diff --git a/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs b/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs
index 55fd7584..0acee656 100644
--- a/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs
+++ b/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs
@@ -310,7 +310,7 @@ public static class CultDocumentMessagePackSerialization
     /// <summary>
     /// Reads a store file: exactly one complete MessagePack array of three slots (header, schema catalog, records), or an empty
     /// array, which is an empty store. A truncated file, bytes after the array, a value that is not an array, a first slot that
-    /// is not a header string, a store with fewer or more slots, a record whose schema the catalog does not publish, or a body
+    /// is not a header string, a store with fewer or more slots, a record whose schema the catalog does not publish, two records under one key, or a body
     /// that does not decode are refused with
     /// <see cref="CultStoreUnreadableException"/>, so a reader never takes part of a file for the whole and never skips a slot
     /// it does not understand. The rewriting paths use this same reader as their verdict on whether a file may be replaced.
@@ -366,6 +366,7 @@ public static class CultDocumentMessagePackSerialization
             }
 
             RequirePublishedSchemas(snapshot);
+            RequireUniqueKeys(snapshot);
             return (snapshot, catalogBytes, recordBytes);
         }
         catch (CultStoreUnreadableException)
@@ -397,6 +398,17 @@ public static class CultDocumentMessagePackSerialization
         }
     }
 
+    // A key names one record: identity is the key string alone, compared by ordinal code units (no case folding, no normalisation).
+    private static void RequireUniqueKeys(CultPersistedStoreSnapshot snapshot)
+    {
+        var seen = new HashSet<string>(StringComparer.Ordinal);
+        foreach (var record in snapshot.Records)
+        {
+            if (!seen.Add(record.Key))
+                throw new CultStoreUnreadableException($"Record '{record.Key}' appears more than once.");
+        }
+    }
+
     // Slot 0 of a non-empty store, read before anything after it: a binary or nil stamp is refused as such, and a string stamp is
     // judged by the caller's rule when it passes one. Bytes that do not open with an array and a string are left to the full read.
     private static void JudgeStamp(byte[] payload, Action<string>? requireReadable)
diff --git a/tests/GameCult.Caching.Tests/StoreReadabilityTests.cs b/tests/GameCult.Caching.Tests/StoreReadabilityTests.cs
index 4aef72a1..3ee64e07 100644
--- a/tests/GameCult.Caching.Tests/StoreReadabilityTests.cs
+++ b/tests/GameCult.Caching.Tests/StoreReadabilityTests.cs
@@ -74,6 +74,18 @@ namespace GameCult.Caching.Tests
             return path;
         }
 
+        [Test]
+        public void ADuplicateKeyIsRefusedNamingTheKeyAndTheFile()
+        {
+            var path = Path.Combine(_directory, "dup.cc");
+            File.WriteAllBytes(path, File.ReadAllBytes(Path.Combine(VectorRoot(), "dup-key-same-schema-v3.bin")));
+
+            Assert.That(() => CultCacheMessagePack.Create(path, new CultCacheOpenOptions { Registry = Registry }),
+                Throws.TypeOf<CultStoreUnreadableException>()
+                    .With.Property(nameof(CultStoreUnreadableException.Path)).EqualTo(path)
+                    .And.Message.Contains("more than once"));
+        }
+
         [TestCaseSource(nameof(Vectors))]
         public void OpenReadsExactlyTheFilesTheVectorsSayItReads(string vector, bool reads)
         {