diff --git a/src/Wiki.Application/Encyclopedia/PlanParsing.cs b/src/Wiki.Application/Encyclopedia/PlanParsing.cs
index 34a2e8d..32e7333 100644
--- a/src/Wiki.Application/Encyclopedia/PlanParsing.cs
+++ b/src/Wiki.Application/Encyclopedia/PlanParsing.cs
@@ -26,12 +26,12 @@ internal static partial class PlanParsing
         return text is not null && text.All(char.IsAsciiLetter) && Enum.TryParse(text, ignoreCase: true, out category);
     }
 
-    /// <summary>The JSON object in a reply, which may be wrapped in a code fence; null when it is not one.</summary>
+    /// <summary>The JSON object in a reply, which may be wrapped in a code fence; null when it is not one or repeats a property.</summary>
     public static JsonObject? Object(string reply)
     {
         try
         {
-            return JsonNode.Parse(StripFence(reply)) as JsonObject;
+            return JsonNode.Parse(StripFence(reply), documentOptions: new JsonDocumentOptions { AllowDuplicateProperties = false }) as JsonObject;
         }
         catch (JsonException)
         {
diff --git a/test/Wiki.Application.Tests/AssignNotesUseCaseTests.cs b/test/Wiki.Application.Tests/AssignNotesUseCaseTests.cs
index 87da135..90b7ea8 100644
--- a/test/Wiki.Application.Tests/AssignNotesUseCaseTests.cs
+++ b/test/Wiki.Application.Tests/AssignNotesUseCaseTests.cs
@@ -170,6 +170,20 @@ public class AssignNotesUseCaseTests
         Assert.Empty(outcome.Assignments);
     }
 
+    [Fact]
+    public async Task Assign_DuplicatePropertyInReply_IsUnusable()
+    {
+        var model = new FakeTextModel().Reply("""
+            {"new":[{"slug":"garden","titleDe":"G","titleEn":"G","titleEn":"H","category":"room","scope":"s"}],
+             "notes":[{"note":"a","articles":["garden"]}]}
+            """);
+
+        var outcome = await UseCase(model).AssignAsync(Batch("a"), [], ArticlePlan.Empty(), CancellationToken.None);
+
+        Assert.True(outcome.Unusable);
+        Assert.Empty(outcome.Assignments);
+    }
+
     [Fact]
     public async Task Assign_PromptIsBounded()
     {