diff --git a/packages/cultcache-ts/src/single-file-messagepack-backing-store.ts b/packages/cultcache-ts/src/single-file-messagepack-backing-store.ts
index b4984913..4fdb2fb6 100644
--- a/packages/cultcache-ts/src/single-file-messagepack-backing-store.ts
+++ b/packages/cultcache-ts/src/single-file-messagepack-backing-store.ts
@@ -103,7 +103,8 @@ export class SingleFileMessagePackBackingStore implements CacheBackingStore {
       }
 
       requireOneTypePerKey(this.filePath, entries, []);
-      await this.#writeAll(entries, []);
+      // A key is held once: of same-type entries sharing a key in one batch, the last is kept.
+      await this.#writeAll([...new Map(entries.map((entry) => [entry.key, entry])).values()], []);
     });
   }
 
diff --git a/packages/cultcache-ts/test/store-file.test.ts b/packages/cultcache-ts/test/store-file.test.ts
index 1ff23320..eb41b77b 100644
--- a/packages/cultcache-ts/test/store-file.test.ts
+++ b/packages/cultcache-ts/test/store-file.test.ts
@@ -623,6 +623,17 @@ test("a pushAll batch that carries one key under two types is refused and writes
   }
 });
 
+test("a pushAll batch that carries one key twice under one type keeps the last and the file reads back with one record", async () => {
+  const directory = await scratch();
+  try {
+    const path = join(directory, "store.cc");
+    await new SingleFileMessagePackBackingStore(path).pushAll([entryOf("k", "t", 1), entryOf("a", "t", 3), entryOf("k", "t", 2)]);
+    assert.deepEqual(await snapshotOf(new SingleFileMessagePackBackingStore(path)), ["t/a:3", "t/k:2"]);
+  } finally {
+    await rm(directory, { recursive: true, force: true });
+  }
+});
+
 test("a legacy envelope array that holds a key under two types is refused on open, as a v1 store is", async () => {
   const directory = await scratch();
   try {