diff --git a/backend/controllers/interviewExperienceController.js b/backend/controllers/interviewExperienceController.js
index b703c733..37f69e5d 100644
--- a/backend/controllers/interviewExperienceController.js
+++ b/backend/controllers/interviewExperienceController.js
@@ -70,7 +70,15 @@ const createInterviewExperience = async (req, res) => {
       payload.color = `hsl(${(payload.company.charCodeAt(0) * 37) % 360}, 55%, 50%)`;
     }
 
-    const existing = await InterviewExperience.findOne({ idempotencyKey });
+    // Scope dedup to userId (authenticated) or clientKey (anonymous).
+    // This prevents one user from retrieving another user's submission by re-using the same key.
+    const dedupFilter = { idempotencyKey };
+    if (payload.userId) {
+      dedupFilter.userId = payload.userId;
+    } else {
+      dedupFilter.clientKey = payload.clientKey;
+    }
+    const existing = await InterviewExperience.findOne(dedupFilter);
     if (existing) {
       return res.status(200).json({
         success: true,
@@ -87,12 +95,16 @@ const createInterviewExperience = async (req, res) => {
       experience: toClientShape(experience),
     });
   } catch (error) {
-    // Concurrent retry won the unique index race — return the first write.
+    // Concurrent retry won the unique index race — return the first write, scoped to the same user/client.
     if (error?.code === 11000 && req.body?.idempotencyKey) {
       try {
-        const existing = await InterviewExperience.findOne({
-          idempotencyKey: req.body.idempotencyKey,
-        });
+        const retryFilter = { idempotencyKey: req.body.idempotencyKey.trim() };
+        if (req.user?._id) {
+          retryFilter.userId = req.user._id;
+        } else if (isSecureClientKey(req.body.clientKey)) {
+          retryFilter.clientKey = req.body.clientKey.trim();
+        }
+        const existing = await InterviewExperience.findOne(retryFilter);
         if (existing) {
           return res.status(200).json({
             success: true,
diff --git a/backend/models/InterviewExperience.js b/backend/models/InterviewExperience.js
index e4344558..edbf1929 100644
--- a/backend/models/InterviewExperience.js
+++ b/backend/models/InterviewExperience.js
@@ -95,12 +95,26 @@ const interviewExperienceSchema = new mongoose.Schema(
   { timestamps: true },
 );
 
+// Compound indexes scope idempotency dedup per-user and per-clientKey, preventing
+// one user from retrieving another user's submission by re-using a key.
 interviewExperienceSchema.index(
-  { idempotencyKey: 1 },
+  { idempotencyKey: 1, userId: 1 },
   {
     unique: true,
     partialFilterExpression: {
       idempotencyKey: { $type: "string", $gt: "" },
+      userId: { $type: "objectId" },
+    },
+  },
+);
+
+interviewExperienceSchema.index(
+  { idempotencyKey: 1, clientKey: 1 },
+  {
+    unique: true,
+    partialFilterExpression: {
+      idempotencyKey: { $type: "string", $gt: "" },
+      clientKey: { $type: "string", $gt: "" },
     },
   },
 );
diff --git a/backend/tests/interviewExperienceController.unit.test.js b/backend/tests/interviewExperienceController.unit.test.js
index 0dbfcbd7..1ec5ce91 100644
--- a/backend/tests/interviewExperienceController.unit.test.js
+++ b/backend/tests/interviewExperienceController.unit.test.js
@@ -164,6 +164,7 @@ describe("createInterviewExperience", () => {
 
     expect(InterviewExperience.findOne).toHaveBeenCalledWith({
       idempotencyKey: "submit-key-abc12345",
+      clientKey: "11111111-1111-4111-8111-111111111111",
     });
     expect(InterviewExperience.create).not.toHaveBeenCalled();
     expect(res.status).toHaveBeenCalledWith(200);