diff --git a/tests/vectors/document-variants-c2a/readability/generate.py b/tests/vectors/document-variants-c2a/readability/generate.py
index c48a5a51..fad89a24 100644
--- a/tests/vectors/document-variants-c2a/readability/generate.py
+++ b/tests/vectors/document-variants-c2a/readability/generate.py
@@ -26,6 +26,8 @@ vectors = {
     "empty-array": pack([]),
     # A legacy envelope array: an array of maps. Rust, TypeScript and Python read it; C# has no legacy reader.
     "legacy-envelopes": pack([{"key": "old", "type": "old.type", "payload": pack([]), "storedAt": "2026-01-01T00:00:00Z"}]),
+    # A legacy envelope array holding key a under two types: a key is one record, whatever the type. Refused like a v3 store with a shared key.
+    "legacy-dup-key": pack([{"key": "a", "type": "old.one", "payload": pack([]), "storedAt": "2026-01-01T00:00:00Z"}, {"key": "a", "type": "old.two", "payload": pack([]), "storedAt": "2026-01-01T00:00:00Z"}]),
     "legacy-trailing": None,  # filled below: the legacy array plus bytes after it
     "ints": pack([1, 2, 3]),
     "nil-first": pack([None, "x"]),
diff --git a/tests/vectors/document-variants-c2a/readability/legacy-dup-key.bin b/tests/vectors/document-variants-c2a/readability/legacy-dup-key.bin
new file mode 100644
index 00000000..c2b117cc
--- /dev/null
+++ b/tests/vectors/document-variants-c2a/readability/legacy-dup-key.bin
@@ -0,0 +1 @@
+’„£key¡a¤type§old.one§payloadÄ
�¨storedAt´2026-01-01T00:00:00Z„£key¡a¤type§old.two§payloadÄ
�¨storedAt´2026-01-01T00:00:00Z
\ No newline at end of file
diff --git a/tests/vectors/document-variants-c2a/readability/manifest.txt b/tests/vectors/document-variants-c2a/readability/manifest.txt
index 96621a10..6d7af19d 100644
--- a/tests/vectors/document-variants-c2a/readability/manifest.txt
+++ b/tests/vectors/document-variants-c2a/readability/manifest.txt
@@ -5,6 +5,7 @@
 zero-byte.bin       refuses  refuses  refuses  refuses
 empty-array.bin     reads    reads    reads    reads
 legacy-envelopes.bin refuses reads    reads    reads
+legacy-dup-key.bin  refuses  refuses  refuses  refuses
 legacy-trailing.bin refuses refuses refuses refuses
 ints.bin            refuses  refuses  refuses  refuses
 nil-first.bin       refuses  refuses  refuses  refuses