Fix online search dedupe and pane layout scrolling

Wino.Core.Domain/Models/MailItem/MailListInitializationOptions.cs

@@ -15,5 +15,6 @@ public record MailListInitializationOptions(IEnumerable<IMailItemFolder> Folders
                                             string SearchQuery,
                                             ConcurrentDictionary<Guid, bool> ExistingUniqueIds = null,
                                             List<MailCopy> PreFetchMailCopies = null,
+                                            bool DeduplicateByServerId = false,
                                             int Skip = 0,
                                             int Take = 0);

Wino.Core.Tests/Services/MailFetchingTests.cs

@@ -55,6 +55,7 @@ public class MailFetchingTests : IAsyncLifetime
             Id = Guid.NewGuid(),
             MailAccountId = _testAccount.Id,
             FolderName = "Inbox",
+            RemoteFolderId = "inbox",
             SpecialFolderType = SpecialFolderType.Inbox,
             IsSystemFolder = true,
             IsSynchronizationEnabled = true
@@ -190,6 +191,112 @@ public class MailFetchingTests : IAsyncLifetime
             "self-sent mail must use account metadata for the sender contact");
     }
 
+    [Fact]
+    public async Task FetchMailsAsync_PreFetchedOnlineSearch_DeduplicatesByServerIdWithinAccount()
+    {
+        var archiveFolder = await CreateFolderAsync(_testAccount, "Archive", "archive", SpecialFolderType.Archive);
+        var sharedId = "server-mail-1";
+        var olderCopy = BuildMail(_inboxFolder.Id, DateTime.UtcNow.AddMinutes(-5));
+        olderCopy.Id = sharedId;
+        var newerCopy = BuildMail(archiveFolder.Id, DateTime.UtcNow);
+        newerCopy.Id = sharedId;
+
+        var options = BuildOptions([_inboxFolder, archiveFolder], createThreads: false, deduplicateByServerId: true) with
+        {
+            PreFetchMailCopies = [olderCopy, newerCopy]
+        };
+
+        var result = await _mailService.FetchMailsAsync(options);
+
+        result.Should().HaveCount(1, "online search should show one visible result per server message within an account");
+        result.Single().UniqueId.Should().Be(newerCopy.UniqueId, "the newest copy should win when the searched folders tie");
+    }
+
+    [Fact]
+    public async Task FetchMailsAsync_PreFetchedOnlineSearch_KeepsSameServerIdAcrossAccountsSeparate()
+    {
+        var secondAccount = await CreateAccountAsync("Second Account", "second@test.local");
+        var secondInbox = await CreateFolderAsync(secondAccount, "Inbox", "inbox-2", SpecialFolderType.Inbox);
+        const string sharedId = "server-mail-2";
+
+        var firstAccountCopy = BuildMail(_inboxFolder.Id, DateTime.UtcNow.AddMinutes(-1));
+        firstAccountCopy.Id = sharedId;
+
+        var secondAccountCopy = BuildMail(secondInbox.Id, DateTime.UtcNow);
+        secondAccountCopy.Id = sharedId;
+
+        var options = BuildOptions([_inboxFolder, secondInbox], createThreads: false, deduplicateByServerId: true) with
+        {
+            PreFetchMailCopies = [firstAccountCopy, secondAccountCopy]
+        };
+
+        var result = await _mailService.FetchMailsAsync(options);
+
+        result.Should().HaveCount(2, "dedupe should be scoped per account, not just per server id string");
+        result.Select(m => m.AssignedAccount!.Id).Should().BeEquivalentTo([_testAccount.Id, secondAccount.Id]);
+    }
+
+    [Fact]
+    public async Task FetchMailsAsync_PreFetchedOnlineSearch_PrefersActiveFolderCopy()
+    {
+        var archiveFolder = await CreateFolderAsync(_testAccount, "Archive", "archive-active", SpecialFolderType.Archive);
+        const string sharedId = "server-mail-3";
+
+        var activeFolderCopy = BuildMail(_inboxFolder.Id, DateTime.UtcNow.AddMinutes(-5));
+        activeFolderCopy.Id = sharedId;
+
+        var newerNonActiveCopy = BuildMail(archiveFolder.Id, DateTime.UtcNow);
+        newerNonActiveCopy.Id = sharedId;
+
+        var options = BuildOptions([_inboxFolder], createThreads: false, deduplicateByServerId: true) with
+        {
+            PreFetchMailCopies = [activeFolderCopy, newerNonActiveCopy]
+        };
+
+        var result = await _mailService.FetchMailsAsync(options);
+
+        result.Should().HaveCount(1);
+        result.Single().FolderId.Should().Be(_inboxFolder.Id, "a copy from the actively searched folder should win over newer non-searched copies");
+    }
+
+    [Fact]
+    public async Task CreateAssignmentAsync_ExistingAssignment_IsIgnored()
+    {
+        var archiveFolder = await CreateFolderAsync(_testAccount, "Archive", "archive-existing", SpecialFolderType.Archive);
+        const string sharedId = "server-mail-4";
+
+        await _databaseService.Connection.InsertAllAsync(new[]
+        {
+            BuildMail(_inboxFolder.Id, DateTime.UtcNow.AddMinutes(-1), id: sharedId),
+            BuildMail(archiveFolder.Id, DateTime.UtcNow, id: sharedId)
+        });
+
+        await _mailService.CreateAssignmentAsync(_testAccount.Id, sharedId, archiveFolder.RemoteFolderId);
+
+        var count = await _databaseService.Connection.Table<MailCopy>().Where(mail => mail.Id == sharedId).CountAsync();
+        count.Should().Be(2, "re-creating an existing folder assignment must not insert another row");
+    }
+
+    [Fact]
+    public async Task CreateAssignmentAsync_NewAssignment_CreatesAdditionalRow()
+    {
+        var archiveFolder = await CreateFolderAsync(_testAccount, "Archive", "archive-new", SpecialFolderType.Archive);
+        const string sharedId = "server-mail-5";
+
+        await _databaseService.Connection.InsertAsync(
+            BuildMail(_inboxFolder.Id, DateTime.UtcNow, id: sharedId),
+            typeof(MailCopy));
+
+        await _mailService.CreateAssignmentAsync(_testAccount.Id, sharedId, archiveFolder.RemoteFolderId);
+
+        var insertedCopies = await _databaseService.Connection.Table<MailCopy>()
+            .Where(mail => mail.Id == sharedId)
+            .ToListAsync();
+
+        insertedCopies.Should().HaveCount(2, "adding a new folder assignment should still clone one additional local row");
+        insertedCopies.Select(mail => mail.FolderId).Should().BeEquivalentTo([_inboxFolder.Id, archiveFolder.Id]);
+    }
+
     // ── Performance: 1 000 mails / ~70 threads ─────────────────────────────────
 
     /// <summary>
@@ -315,12 +422,13 @@ public class MailFetchingTests : IAsyncLifetime
         Guid folderId,
         DateTime creationDate,
         string? threadId = null,
-        string fromAddress = "external@example.com")
+        string fromAddress = "external@example.com",
+        string? id = null)
     {
         return new MailCopy
         {
             UniqueId = Guid.NewGuid(),
-            Id = Guid.NewGuid().ToString(),
+            Id = id ?? Guid.NewGuid().ToString(),
             FileId = Guid.NewGuid(),
             FolderId = folderId,
             Subject = $"Subject {Guid.NewGuid():N}",
@@ -336,7 +444,8 @@ public class MailFetchingTests : IAsyncLifetime
     private static MailListInitializationOptions BuildOptions(
         IEnumerable<MailItemFolder> folders,
         bool createThreads = true,
-        int take = 0)
+        int take = 0,
+        bool deduplicateByServerId = false)
     {
         return new MailListInitializationOptions(
             Folders: folders,
@@ -345,9 +454,42 @@ public class MailFetchingTests : IAsyncLifetime
             CreateThreads: createThreads,
             IsFocusedOnly: null,
             SearchQuery: null,
+            DeduplicateByServerId: deduplicateByServerId,
             Take: take);
     }
 
+    private async Task<MailAccount> CreateAccountAsync(string name, string address)
+    {
+        var account = new MailAccount
+        {
+            Id = Guid.NewGuid(),
+            Name = name,
+            Address = address,
+            SenderName = name,
+            ProviderType = MailProviderType.IMAP4
+        };
+
+        await _databaseService.Connection.InsertAsync(account, typeof(MailAccount));
+        return account;
+    }
+
+    private async Task<MailItemFolder> CreateFolderAsync(MailAccount account, string name, string remoteFolderId, SpecialFolderType specialFolderType)
+    {
+        var folder = new MailItemFolder
+        {
+            Id = Guid.NewGuid(),
+            MailAccountId = account.Id,
+            FolderName = name,
+            RemoteFolderId = remoteFolderId,
+            SpecialFolderType = specialFolderType,
+            IsSystemFolder = true,
+            IsSynchronizationEnabled = true
+        };
+
+        await _databaseService.Connection.InsertAsync(folder, typeof(MailItemFolder));
+        return folder;
+    }
+
     /// <summary>
     /// Builds a MailService wired to real FolderService, AccountService, and ContactService
     /// all backed by the shared in-memory database, so the full SQL batch path is exercised.

Wino.Core/Synchronizers/GmailSynchronizer.cs

@@ -1390,6 +1390,12 @@ public class GmailSynchronizer : WinoSynchronizer<IClientServiceRequest, Message
         static bool IsArchiveFolder(IMailItemFolder folder)
             => folder?.SpecialFolderType == SpecialFolderType.Archive || folder?.RemoteFolderId == ServiceConstants.ARCHIVE_LABEL_ID;
 
+        var distinctFolders = folders?
+            .Where(folder => folder != null)
+            .GroupBy(folder => folder.Id)
+            .Select(group => group.First())
+            .ToList();
+
         var messageIds = new HashSet<string>(StringComparer.Ordinal);
 
         async Task CollectMessageIdsAsync(UsersResource.MessagesResource.ListRequest request)
@@ -1421,7 +1427,7 @@ public class GmailSynchronizer : WinoSynchronizer<IClientServiceRequest, Message
         bool hasScopedQuery = queryText.StartsWith("label:", StringComparison.OrdinalIgnoreCase) ||
                               queryText.StartsWith("in:", StringComparison.OrdinalIgnoreCase);
 
-        if (hasScopedQuery || folders?.Count == 0)
+        if (hasScopedQuery || distinctFolders?.Count == 0)
         {
             var request = _gmailService.Users.Messages.List("me");
             request.Q = queryText;
@@ -1431,7 +1437,7 @@ public class GmailSynchronizer : WinoSynchronizer<IClientServiceRequest, Message
         }
         else
         {
-            foreach (var folder in folders)
+            foreach (var folder in distinctFolders)
             {
                 cancellationToken.ThrowIfCancellationRequested();
 

Wino.Core/Synchronizers/ImapSynchronizer.cs

@@ -1053,10 +1053,15 @@ public class ImapSynchronizer : WinoSynchronizer<ImapRequest, ImapMessageCreatio
         {
             client = await _clientPool.GetClientAsync().ConfigureAwait(false);
 
-            List<MailCopy> searchResults = [];
+            var distinctFolders = folders?
-            List<string> searchResultFolderMailUids = [];
+                .Where(folder => folder != null)
+                .GroupBy(folder => folder.Id)
+                .Select(group => group.First())
+                .ToList() ?? [];
 
-            foreach (var folder in folders)
+            HashSet<string> searchResultFolderMailUids = new(StringComparer.Ordinal);
+
+            foreach (var folder in distinctFolders)
             {
                 if (folder is not MailItemFolder localFolder)
                     continue;

Wino.Core/Synchronizers/OutlookSynchronizer.cs

@@ -267,18 +267,32 @@ public class OutlookSynchronizer : WinoSynchronizer<RequestInformation, Message,
         return MailSynchronizationResult.CompletedWithFolderResults(unreadNewItems, folderResults);
     }
 
-    public async Task DownloadSearchResultMessageAsync(string messageId, MailItemFolder assignedFolder, CancellationToken cancellationToken = default)
+    public Task DownloadSearchResultMessageAsync(string messageId, MailItemFolder assignedFolder, CancellationToken cancellationToken = default)
+        => DownloadSearchResultMessageAsync(messageId, assignedFolder, existingMessageIds: null, cancellationToken);
+
+    private async Task DownloadSearchResultMessageAsync(string messageId,
+                                                        MailItemFolder assignedFolder,
+                                                        ISet<string> existingMessageIds,
+                                                        CancellationToken cancellationToken = default)
     {
         if (string.IsNullOrWhiteSpace(messageId) || assignedFolder == null) return;
 
         // Online search can return the same message across repeated invocations/races.
         // Guard before network+MIME download and before database insert.
-        var existing = await _outlookChangeProcessor.AreMailsExistsAsync([messageId]).ConfigureAwait(false);
+        if (existingMessageIds?.Contains(messageId) == true)
-        if (existing.Contains(messageId))
         {
             return;
         }
 
+        if (existingMessageIds == null)
+        {
+            var existing = await _outlookChangeProcessor.AreMailsExistsAsync([messageId]).ConfigureAwait(false);
+            if (existing.Contains(messageId))
+            {
+                return;
+            }
+        }
+
         Log.Information("Downloading search result message {messageId} for {Name} - {FolderName}", messageId, Account.Name, assignedFolder.FolderName);
 
         // Outlook message handling was a little strange.
@@ -314,6 +328,8 @@ public class OutlookSynchronizer : WinoSynchronizer<RequestInformation, Message,
             // Use safe upsert path to avoid duplicate rows when message already exists.
             await _outlookChangeProcessor.CreateMailAsync(Account.Id, package).ConfigureAwait(false);
         }
+
+        existingMessageIds?.Add(messageId);
     }
 
     private async Task<IEnumerable<string>> SynchronizeFolderAsync(MailItemFolder folder, CancellationToken cancellationToken = default)
@@ -2226,10 +2242,11 @@ public class OutlookSynchronizer : WinoSynchronizer<RequestInformation, Message,
         if (messageIdsWithKnownFolder.Count == 0) return [];
 
         var locallyExistingMails = await _outlookChangeProcessor.AreMailsExistsAsync(messageIdsWithKnownFolder).ConfigureAwait(false);
+        var existingMessageIds = new HashSet<string>(locallyExistingMails, StringComparer.Ordinal);
 
         // Find messages that are not downloaded yet.
         List<Message> messagesToDownload = [];
-        foreach (var id in messageIdsWithKnownFolder.Except(locallyExistingMails, StringComparer.Ordinal))
+        foreach (var id in messageIdsWithKnownFolder.Except(existingMessageIds, StringComparer.Ordinal))
         {
             if (messagesById.TryGetValue(id, out var message))
             {
@@ -2239,7 +2256,7 @@ public class OutlookSynchronizer : WinoSynchronizer<RequestInformation, Message,
 
         foreach (var message in messagesToDownload)
         {
-            await DownloadSearchResultMessageAsync(message.Id, localFolders[message.ParentFolderId], cancellationToken).ConfigureAwait(false);
+            await DownloadSearchResultMessageAsync(message.Id, localFolders[message.ParentFolderId], existingMessageIds, cancellationToken).ConfigureAwait(false);
         }
 
         // Get results from database and return.

Wino.Mail.ViewModels/MailListPageViewModel.cs

@@ -1084,7 +1084,13 @@ public partial class MailListPageViewModel : MailBaseViewModel,
     {
         if (handlingFolders == null) return [];
 
-        var foldersByAccount = handlingFolders
+        var distinctFolders = handlingFolders
+            .Where(folder => folder != null)
+            .GroupBy(folder => folder.Id)
+            .Select(group => group.First())
+            .ToList();
+
+        var foldersByAccount = distinctFolders
             .GroupBy(a => a.MailAccountId)
             .ToList();
 
@@ -1101,13 +1107,44 @@ public partial class MailListPageViewModel : MailBaseViewModel,
 
         var allResults = await Task.WhenAll(searchTasks).ConfigureAwait(false);
 
-        return allResults
+        var accountIdsByFolderId = distinctFolders.ToDictionary(folder => folder.Id, folder => folder.MailAccountId);
-            .SelectMany(a => a)
+        var preferredFolderIds = distinctFolders.Select(folder => folder.Id).ToHashSet();
-            .GroupBy(a => a.UniqueId)
+
-            .Select(a => a.First())
+        return DeduplicateOnlineSearchResults(allResults.SelectMany(a => a), accountIdsByFolderId, preferredFolderIds);
+    }
+
+    private static List<MailCopy> DeduplicateOnlineSearchResults(IEnumerable<MailCopy> results,
+                                                                 IReadOnlyDictionary<Guid, Guid> accountIdsByFolderId,
+                                                                 ISet<Guid> preferredFolderIds)
+    {
+        if (results == null) return [];
+
+        return results
+            .Where(mail => mail != null)
+            .GroupBy(mail => (ResolveMailAccountId(mail, accountIdsByFolderId), ResolveSearchMailId(mail)))
+            .Select(group => group
+                .OrderByDescending(mail => preferredFolderIds.Contains(mail.FolderId))
+                .ThenByDescending(mail => mail.CreationDate)
+                .ThenBy(mail => mail.FolderId)
+                .ThenBy(mail => mail.UniqueId)
+                .First())
             .ToList();
     }
 
+    private static Guid ResolveMailAccountId(MailCopy mail, IReadOnlyDictionary<Guid, Guid> accountIdsByFolderId)
+    {
+        if (mail?.AssignedAccount != null)
+            return mail.AssignedAccount.Id;
+
+        if (mail != null && accountIdsByFolderId.TryGetValue(mail.FolderId, out var accountId))
+            return accountId;
+
+        return Guid.Empty;
+    }
+
+    private static string ResolveSearchMailId(MailCopy mail)
+        => string.IsNullOrWhiteSpace(mail?.Id) ? mail?.UniqueId.ToString("N") ?? string.Empty : mail.Id;
+
     private async Task InitializeFolderAsync()
     {
         if (SelectedFilterOption == null || SelectedFolderPivot == null || SelectedSortingOption == null)
@@ -1188,7 +1225,8 @@ public partial class MailListPageViewModel : MailBaseViewModel,
                                                                           SelectedFolderPivot.IsFocused,
                                                                           isDoingOnlineSearch ? string.Empty : SearchQuery,
                                                                           MailCollection.MailCopyIdHashSet,
-                                                                          onlineSearchItems);
+                                                                          onlineSearchItems,
+                                                                          DeduplicateByServerId: isDoingOnlineSearch);
 
             items = await _mailService.FetchMailsAsync(initializationOptions, cancellationToken).ConfigureAwait(false);
 

Wino.Mail.WinUI/Views/WinoAppShell.xaml

@@ -642,22 +642,27 @@
                         </ListView>
                     </Grid>
 
-                    <StackPanel
+                    <ScrollViewer
                         x:Name="ContactsPaneContent"
                         Margin="20,20,16,0"
-                        Spacing="6"
+                        HorizontalScrollBarVisibility="Disabled"
+                        HorizontalScrollMode="Disabled"
+                        VerticalScrollBarVisibility="Auto"
+                        VerticalScrollMode="Auto"
                         Visibility="Collapsed">
-                        <TextBlock
+                        <StackPanel Spacing="6">
-                            FontSize="16"
+                            <TextBlock
-                            FontWeight="SemiBold"
+                                FontSize="16"
-                            Text="{x:Bind domain:Translator.ContactsPane_DescriptionTitle, Mode=OneTime}"
+                                FontWeight="SemiBold"
-                            TextWrapping="WrapWholeWords" />
+                                Text="{x:Bind domain:Translator.ContactsPane_DescriptionTitle, Mode=OneTime}"
-                        <TextBlock
+                                TextWrapping="WrapWholeWords" />
-                            Foreground="{ThemeResource TextFillColorSecondaryBrush}"
+                            <TextBlock
-                            Style="{StaticResource BodyTextBlockStyle}"
+                                Foreground="{ThemeResource TextFillColorSecondaryBrush}"
-                            Text="{x:Bind domain:Translator.ContactsPane_DescriptionBody, Mode=OneTime}"
+                                Style="{StaticResource BodyTextBlockStyle}"
-                            TextWrapping="WrapWholeWords" />
+                                Text="{x:Bind domain:Translator.ContactsPane_DescriptionBody, Mode=OneTime}"
-                    </StackPanel>
+                                TextWrapping="WrapWholeWords" />
+                        </StackPanel>
+                    </ScrollViewer>
                 </Grid>
             </muxc:NavigationView.PaneCustomContent>
             <Grid ColumnSpacing="0">

Wino.Mail.WinUI/Views/WinoAppShell.xaml.cs

@@ -29,6 +29,7 @@ using Wino.Mail.ViewModels.Data;
 using Wino.Mail.WinUI.ViewModels;
 using Wino.Mail.WinUI.Controls;
 using Wino.Mail.WinUI.Helpers;
+using Wino.Helpers;
 using Wino.MenuFlyouts;
 using Wino.MenuFlyouts.Context;
 using Wino.Messaging.Client.Accounts;
@@ -50,10 +51,15 @@ public sealed partial class WinoAppShell : Views.Abstract.WinoAppShellAbstract,
 {
     private const string StateDefaultShellContent = "DefaultShellContentState";
     private const string StateEventDetailsContent = "EventDetailsContentState";
+    private const int PaneCustomContentRowIndex = 4;
+    private const int PaneItemsContainerRowIndex = 6;
     private WinoApplicationMode? _activeMode;
     private bool _isSyncingNavigationViewSelection;
     private bool _isSynchronizingVisibleDateRangeCalendar;
     private bool _isPreparedForWindowClose;
+    private Grid? _paneContentGrid;
+    private RowDefinition? _paneCustomContentRowDefinition;
+    private RowDefinition? _paneItemsContainerRowDefinition;
 
     public WinoAppShell()
     {
@@ -681,6 +687,22 @@ public sealed partial class WinoAppShell : Views.Abstract.WinoAppShellAbstract,
 
     private void UpdateNavigationPaneLayout(NavigationViewDisplayMode displayMode)
     {
+        EnsureNavigationPaneLayoutParts();
+
+        bool shouldStretchCustomPane = displayMode == NavigationViewDisplayMode.Expanded
+                                       && navigationView.IsPaneOpen
+                                       && (ViewModel.IsCalendarMode || ViewModel.IsContactsMode);
+
+        if (_paneCustomContentRowDefinition != null && _paneItemsContainerRowDefinition != null)
+        {
+            _paneCustomContentRowDefinition.Height = shouldStretchCustomPane
+                ? new GridLength(1, GridUnitType.Star)
+                : GridLength.Auto;
+            _paneItemsContainerRowDefinition.Height = shouldStretchCustomPane
+                ? GridLength.Auto
+                : new GridLength(1, GridUnitType.Star);
+        }
+
         if (displayMode == NavigationViewDisplayMode.Expanded && navigationView.IsPaneOpen)
         {
             if (ViewModel.IsCalendarMode)
@@ -710,6 +732,17 @@ public sealed partial class WinoAppShell : Views.Abstract.WinoAppShellAbstract,
             : new Thickness(0);
     }
 
+    private void EnsureNavigationPaneLayoutParts()
+    {
+        _paneContentGrid ??= WinoVisualTreeHelper.GetChildObject<Grid>(navigationView, "PaneContentGrid");
+
+        if (_paneContentGrid == null || _paneContentGrid.RowDefinitions.Count <= PaneItemsContainerRowIndex)
+            return;
+
+        _paneCustomContentRowDefinition ??= _paneContentGrid.RowDefinitions[PaneCustomContentRowIndex];
+        _paneItemsContainerRowDefinition ??= _paneContentGrid.RowDefinitions[PaneItemsContainerRowIndex];
+    }
+
     private async void OnPreviewKeyDown(object sender, KeyRoutedEventArgs e)
     {
         if (e.KeyStatus.RepeatCount > 1 || ShouldIgnoreShortcut())

Wino.Services/MailService.cs

@@ -246,11 +246,13 @@ public class MailService : BaseDatabaseService, IMailService
     private static List<MailCopy> ApplyOptionsToPreFetchedMails(MailListInitializationOptions options)
     {
         var allowedFolderIds = options.Folders.Select(f => f.Id).ToHashSet();
+        var accountIdsByFolderId = options.Folders
+            .Where(folder => folder != null)
+            .GroupBy(folder => folder.Id)
+            .ToDictionary(group => group.Key, group => group.First().MailAccountId);
 
         IEnumerable<MailCopy> query = options.PreFetchMailCopies
-            .Where(m => m != null && allowedFolderIds.Contains(m.FolderId))
+            .Where(m => m != null && allowedFolderIds.Contains(m.FolderId));
-            .GroupBy(m => m.UniqueId)
-            .Select(g => g.First());
 
         switch (options.FilterType)
         {
@@ -285,6 +287,19 @@ public class MailService : BaseDatabaseService, IMailService
             query = query.Where(m => !options.ExistingUniqueIds.ContainsKey(m.UniqueId));
         }
 
+        query = options.DeduplicateByServerId
+            ? query
+                .GroupBy(m => (ResolveMailAccountId(m, accountIdsByFolderId), ResolveServerMailId(m)))
+                .Select(group => group
+                    .OrderByDescending(m => allowedFolderIds.Contains(m.FolderId))
+                    .ThenByDescending(m => m.CreationDate)
+                    .ThenBy(m => m.FolderId)
+                    .ThenBy(m => m.UniqueId)
+                    .First())
+            : query
+                .GroupBy(m => m.UniqueId)
+                .Select(group => group.First());
+
         query = options.SortingOptionType switch
         {
             SortingOptionType.Sender => query.OrderBy(m => m.FromName).ThenByDescending(m => m.CreationDate),
@@ -304,6 +319,20 @@ public class MailService : BaseDatabaseService, IMailService
         return query.ToList();
     }
 
+    private static Guid ResolveMailAccountId(MailCopy mail, IReadOnlyDictionary<Guid, Guid> accountIdsByFolderId)
+    {
+        if (mail?.AssignedAccount != null)
+            return mail.AssignedAccount.Id;
+
+        if (mail != null && accountIdsByFolderId.TryGetValue(mail.FolderId, out var accountId))
+            return accountId;
+
+        return Guid.Empty;
+    }
+
+    private static string ResolveServerMailId(MailCopy mail)
+        => string.IsNullOrWhiteSpace(mail?.Id) ? mail?.UniqueId.ToString("N") ?? string.Empty : mail.Id;
+
     public async Task<List<MailCopy>> FetchMailsAsync(MailListInitializationOptions options, CancellationToken cancellationToken = default)
     {
         List<MailCopy> mails;
@@ -777,6 +806,13 @@ public class MailService : BaseDatabaseService, IMailService
             return;
         }
 
+        if (await IsMailExistsAsync(mailCopyId, localFolder.Id).ConfigureAwait(false))
+        {
+            _logger.Debug("Skipping assignment creation for {MailCopyId} because folder {FolderId} already has a local copy.",
+                mailCopyId, localFolder.Id);
+            return;
+        }
+
         var mailCopy = await GetSingleMailItemWithoutFolderAssignmentAsync(mailCopyId);
 
         if (mailCopy == null)