diff --git a/LehrerApp.Desktop.Tests/WebUntisRestSyncProcessorTests.cs b/LehrerApp.Desktop.Tests/WebUntisRestSyncProcessorTests.cs
index 41e8bf1..2dbbd24 100644
--- a/LehrerApp.Desktop.Tests/WebUntisRestSyncProcessorTests.cs
+++ b/LehrerApp.Desktop.Tests/WebUntisRestSyncProcessorTests.cs
@@ -61,6 +61,75 @@ public sealed class WebUntisRestSyncProcessorTests
         Assert.Empty(exams.GetAll());
     }
 
+    [Fact]
+    public void Process_DoppeltAngelegteArbeitDurchGeraeteSync_WirdAufEinenDatensatzReduziert()
+    {
+        var group = new LearningGroup { Id = Guid.NewGuid(), Name = "Mat_E" };
+        var mappings = new FakeUntisSlotMappings();
+        mappings.Add(new UntisSlotMapping
+        {
+            Weekday = DayOfWeek.Tuesday, StartTime = new TimeOnly(9, 40), Summary = "Mat_E",
+            ClassToken = "10a", Kind = SubstitutionKind.Lesson, GroupId = group.Id,
+            PeriodNumber = 3, CoveredPeriods = [3, 4], Confirmed = true,
+        });
+        var exam = Entry([2002979, 2002982], "2026-09-29T09:40", "2026-09-29T11:10",
+            "EXAM", "REGULAR", "Mat_E", "10a", "Arbeit 1. Mathearbeit");
+        var externalId = WebUntisRestSyncProcessor.ExamPrefix + "2002979-2002982";
+        // Zwei Geräte haben dieselbe WebUntis-Arbeit unabhängig voneinander (vor dem gegenseitigen
+        // Sync) mit derselben ExternalId, aber unterschiedlicher Id angelegt.
+        var olderDuplicate = new Exam
+        {
+            GroupId = group.Id, Date = new DateOnly(2026, 9, 29), Title = "Arbeit 1. Mathearbeit",
+            ExternalId = externalId, CreatedAt = new DateTime(2026, 9, 22, 8, 0, 0, DateTimeKind.Utc),
+        };
+        var newerDuplicate = new Exam
+        {
+            GroupId = group.Id, Date = new DateOnly(2026, 9, 29), Title = "Arbeit 1. Mathearbeit",
+            ExternalId = externalId, CreatedAt = new DateTime(2026, 9, 22, 9, 0, 0, DateTimeKind.Utc),
+        };
+        var exams = new RecordingExams([olderDuplicate, newerDuplicate]);
+        var processor = BuildProcessor(mappings, new FakeSubstitutionEntries(), exams, [group]);
+
+        var result = processor.Process([exam], Monday, Monday.AddDays(13), Monday.AddDays(27));
+
+        var remaining = Assert.Single(exams.GetAll());
+        Assert.Equal(olderDuplicate.Id, remaining.Id);
+        Assert.Equal(1, result.ExamChanges);
+    }
+
+    [Fact]
+    public void Process_DupliziertMitBereitsBenoteterKlausur_LoeschtNurDenNochPlanentenDatensatz()
+    {
+        var group = new LearningGroup { Id = Guid.NewGuid(), Name = "Mat_E" };
+        var mappings = new FakeUntisSlotMappings();
+        mappings.Add(new UntisSlotMapping
+        {
+            Weekday = DayOfWeek.Tuesday, StartTime = new TimeOnly(9, 40), Summary = "Mat_E",
+            ClassToken = "10a", Kind = SubstitutionKind.Lesson, GroupId = group.Id,
+            PeriodNumber = 3, CoveredPeriods = [3, 4], Confirmed = true,
+        });
+        var exam = Entry([2002979, 2002982], "2026-09-29T09:40", "2026-09-29T11:10",
+            "EXAM", "REGULAR", "Mat_E", "10a", "Arbeit 1. Mathearbeit");
+        var externalId = WebUntisRestSyncProcessor.ExamPrefix + "2002979-2002982";
+        var gradedDuplicate = new Exam
+        {
+            GroupId = group.Id, Date = new DateOnly(2026, 9, 29), Title = "Arbeit 1. Mathearbeit",
+            ExternalId = externalId, Status = ExamStatus.Graded,
+        };
+        var plannedDuplicate = new Exam
+        {
+            GroupId = group.Id, Date = new DateOnly(2026, 9, 29), Title = "Arbeit 1. Mathearbeit",
+            ExternalId = externalId,
+        };
+        var exams = new RecordingExams([gradedDuplicate, plannedDuplicate]);
+        var processor = BuildProcessor(mappings, new FakeSubstitutionEntries(), exams, [group]);
+
+        processor.Process([exam], Monday, Monday.AddDays(13), Monday.AddDays(27));
+
+        var remaining = Assert.Single(exams.GetAll());
+        Assert.Equal(gradedDuplicate.Id, remaining.Id);
+    }
+
     [Fact]
     public void Process_ManuelleArbeitAmSelbenTag_WirdNichtDupliziertOderGeloescht()
     {
diff --git a/LehrerApp.Desktop/Services/WebUntisRestSyncProcessor.cs b/LehrerApp.Desktop/Services/WebUntisRestSyncProcessor.cs
index 709140a..bdf4396 100644
--- a/LehrerApp.Desktop/Services/WebUntisRestSyncProcessor.cs
+++ b/LehrerApp.Desktop/Services/WebUntisRestSyncProcessor.cs
@@ -138,6 +138,7 @@ public sealed class WebUntisRestSyncProcessor(
         var changes = 0;
         var desiredIds = new HashSet<string>();
         var allExams = exams.GetAll();
+        changes += DeduplicateExams(allExams);
         foreach (var entry in examEntries)
         {
             var periods = ResolveCoveredPeriods(entry.Start, entry.End);
@@ -185,6 +186,34 @@ public sealed class WebUntisRestSyncProcessor(
         return changes;
     }
 
+    /// <summary>Räumt Klausuren auf, die zwei Geräte unabhängig voneinander mit derselben
+    /// <see cref="Exam.ExternalId"/> aber unterschiedlicher <see cref="Exam.Id"/> angelegt haben,
+    /// weil jedes Gerät den anderen Datensatz zum Zeitpunkt seines eigenen Polls noch nicht
+    /// gesynct hatte (Race Condition zwischen WebUntis-REST-Abgleich und Geräte-Sync). Pro Gruppe
+    /// gleicher ExternalId gewinnt der am weitesten bearbeitete Datensatz (nicht mehr "Planned"),
+    /// sonst der zuerst angelegte; bereits bearbeitete Duplikate werden nie automatisch gelöscht,
+    /// um keine eingetragenen Ergebnisse zu verlieren - dieselbe Zurückhaltung wie beim
+    /// Stale-Abbau unten (nur `Status == Planned` wird automatisch entfernt).</summary>
+    private int DeduplicateExams(List<Exam> allExams)
+    {
+        var changes = 0;
+        foreach (var group in allExams.Where(exam => exam.ExternalId is not null)
+                     .GroupBy(exam => exam.ExternalId)
+                     .Where(group => group.Count() > 1))
+        {
+            var ordered = group.OrderBy(exam => exam.Status == ExamStatus.Planned ? 1 : 0)
+                .ThenBy(exam => exam.CreatedAt).ToList();
+            foreach (var duplicate in ordered.Skip(1))
+            {
+                if (duplicate.Status != ExamStatus.Planned) continue;
+                exams.Delete(duplicate.Id);
+                allExams.Remove(duplicate);
+                changes++;
+            }
+        }
+        return changes;
+    }
+
     private UntisSlotMapping? FindMapping(List<UntisSlotMapping> candidates, DayOfWeek weekday, TimeOnly start,
         List<int> periods, UntisTimetableGridEntryDto entry)
     {