diff --git a/src/server/turn-service.ts b/src/server/turn-service.ts
index 39d8728..e999669 100644
--- a/src/server/turn-service.ts
+++ b/src/server/turn-service.ts
@@ -55,9 +55,23 @@ export function buildTeacherContext(graphStore: GraphStore, chatId: string): str
  * NEVER touched (the client's next POST still carries them verbatim).
  */
 export function withMessageIds(messages: UIMessage[]): UIMessage[] {
-	return messages.map((m) =>
+	const filled = messages.map((m) =>
 		m.id ? m : { ...m, id: `msg_${Date.now().toString(36)}_${crypto.randomUUID().slice(0, 8)}` },
 	);
+	// Same id twice = the SDK's continuation merge couldn't happen in place
+	// (continuation coalescing appends a user-role steer AFTER the assistant
+	// message being continued, so onFinish carries the input copy AND the
+	// extended response copy under one id). Keep the LAST occurrence — it is
+	// the SDK's final merge state (a superset of the earlier copy).
+	const seen = new Set<string>();
+	const deduped: UIMessage[] = [];
+	for (let i = filled.length - 1; i >= 0; i--) {
+		const m = filled[i]!;
+		if (seen.has(m.id)) continue;
+		seen.add(m.id);
+		deduped.unshift(m);
+	}
+	return deduped;
 }
 
 export function persistTurn(chatStore: ChatStore, chatId: string, messages: UIMessage[], isAborted: boolean): void {
diff --git a/tests/jobs.test.ts b/tests/jobs.test.ts
index ba0c107..b2382a1 100644
--- a/tests/jobs.test.ts
+++ b/tests/jobs.test.ts
@@ -764,6 +764,33 @@ describe("persistTurn id hygiene", () => {
 		assert.notEqual(filled[0]!.id, filled[1]!.id);
 	});
 
+	it("persistTurn dedupes same-id continuation copies, keeping the LAST (the SDK's merged superset)", () => {
+		// The coalescing shape: a steer (user-role) rides a quiz-answer
+		// continuation AFTER the assistant message being continued, so the
+		// SDK's onFinish list carries the input copy AND the extended
+		// response copy under one id — the record must keep only the merge.
+		const chat = chatStore.createChat("Test: dedupe continuation copies");
+		trackChat(chat.id);
+		const short = { id: "msg_dup_1", role: "assistant", parts: [{ type: "text", text: "step 1" }] };
+		const long = { id: "msg_dup_1", role: "assistant", parts: [{ type: "text", text: "step 1" }, { type: "text", text: "step 2 (continued)" }] };
+		persistTurn(chatStore, chat.id, [
+			{ id: "u1", role: "user", parts: [{ type: "text", text: "hi" }] },
+			short,
+			{ id: "steer_x", role: "user", parts: [{ type: "text", text: "research complete" }] },
+			long,
+		], false);
+		const record = chatStore.getChat(chat.id)!;
+		assert.equal(record.messages.length, 3, "the duplicate entry is dropped");
+		assert.deepEqual(
+			record.messages.map((m) => m.id),
+			["u1", "steer_x", "msg_dup_1"],
+			"order preserved, steer before the merged assistant copy",
+		);
+		const merged = record.messages[2]!;
+		assert.equal(merged.parts.length, 2, "the KEPT copy is the extended one");
+		assert.equal((merged.parts[1] as { text: string }).text, "step 2 (continued)");
+	});
+
 	it("persistTurn saves every message with a unique non-empty id (wake turns never re-POSTed)", () => {
 		const chat = chatStore.createChat("Test: persist ids");
 		trackChat(chat.id);