diff --git a/desktop/src/features/projects/useAttachProjectRepository.test.mjs b/desktop/src/features/projects/useAttachProjectRepository.test.mjs
new file mode 100644
index 0000000000..8ae35b1584
--- /dev/null
+++ b/desktop/src/features/projects/useAttachProjectRepository.test.mjs
@@ -0,0 +1,55 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { attachProjectRepository } from "./useAttachProjectRepository.ts";
+
+const OWNER = "a".repeat(64);
+const CACHED_BASE = "e".repeat(64);
+
+function project() {
+  return {
+    baseRevisionId: CACHED_BASE,
+    createdAt: 100,
+    dtag: "platform",
+    effectiveRevisionId: CACHED_BASE,
+    id: `30621:${OWNER}:platform`,
+    owner: OWNER,
+    projectAddress: `30621:${OWNER}:platform`,
+    relatedChannelIds: [],
+  };
+}
+
+const repository = {
+  id: `${OWNER}:mobile`,
+  dtag: "mobile",
+  owner: OWNER,
+  repoAddress: `30617:${OWNER}:mobile`,
+};
+
+test("attachProjectRepository rejects an equal-timestamp base replacement", async () => {
+  let revisionHeadFetches = 0;
+  await assert.rejects(
+    attachProjectRepository(
+      { project: project(), repository },
+      {
+        fetchEvents: async () => [
+          {
+            id: "d".repeat(64),
+            kind: 30621,
+            pubkey: OWNER,
+            created_at: 100,
+            content: "",
+            tags: [["d", "platform"]],
+            sig: "0".repeat(128),
+          },
+        ],
+        fetchProjectRevisionHeads: async () => {
+          revisionHeadFetches += 1;
+          return [];
+        },
+      },
+    ),
+    /updated by another session/,
+  );
+  assert.equal(revisionHeadFetches, 1);
+});