diff --git a/migrations/361_attempt_session_question_unique.sql b/migrations/361_attempt_session_question_unique.sql
new file mode 100644
index 0000000..5670886
--- /dev/null
+++ b/migrations/361_attempt_session_question_unique.sql
@@ -0,0 +1,14 @@
+-- 361: quiz 세션 내 같은 문제 이중 attempt 방지 partial UNIQUE (R9).
+-- submit_attempt 의 FOR UPDATE 행잠금이 1차 방어이고, 이 제약은 DB 레벨 belt-and-suspenders
+-- (모바일 더블탭/재시도가 어떤 경로로든 이중 INSERT 에 도달해도 차단). prod 실측 중복 0 건
+-- (SELECT ... GROUP BY HAVING count>1 = 0) — dedup DELETE 는 멱등 precaution, UNIQUE 는 안전.
+-- quiz_session_id IS NULL(세션 외 직접 입력)은 대상 아님 → partial index.
+DELETE FROM study_question_attempts a USING study_question_attempts b
+WHERE a.quiz_session_id IS NOT NULL
+  AND a.quiz_session_id = b.quiz_session_id
+  AND a.study_question_id = b.study_question_id
+  AND a.id > b.id;
+
+CREATE UNIQUE INDEX IF NOT EXISTS uq_attempt_session_question
+ON study_question_attempts (quiz_session_id, study_question_id)
+WHERE quiz_session_id IS NOT NULL;