diff --git a/backend/app/routers/auth.py b/backend/app/routers/auth.py
index 79a5f91..61e99ae 100644
--- a/backend/app/routers/auth.py
+++ b/backend/app/routers/auth.py
@@ -66,8 +66,15 @@ class UserResponse(BaseModel):
 # --- Helpers ---
 
 def _create_access_token(data: dict) -> str:
-    """Create a signed access token using RS256."""
+    """Create a signed access token using RS256.
+
+    Includes a unique ``jti`` claim so that concurrent/same-second calls
+    (e.g. back-to-back /refresh requests) still produce distinct tokens —
+    without it, two tokens minted for the same user within the same
+    second are byte-identical since ``sub``/``exp`` are the only claims.
+    """
     to_encode = data.copy()
+    to_encode["jti"] = str(uuid.uuid4())
     expire = datetime.now(timezone.utc) + timedelta(
         minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES
     )
diff --git a/backend/tests/test_refresh_endpoint.py b/backend/tests/test_refresh_endpoint.py
index 995bf0f..92e4322 100644
--- a/backend/tests/test_refresh_endpoint.py
+++ b/backend/tests/test_refresh_endpoint.py
@@ -67,7 +67,7 @@ class TestRefresh:
 
     def test_refresh_success(self):
         """A valid refresh token returns a new access token."""
-        refresh_token = self._register_user("refreshuser", "secret123")
+        refresh_token = self._register_user("refep_refreshuser", "secret123")
         client = TestClient(app)
         resp = client.post(
             "/api/auth/refresh",
@@ -82,7 +82,7 @@ class TestRefresh:
 
     def test_refresh_returns_different_access_token(self):
         """Refreshing produces a new access token (different from any previous)."""
-        refresh_token = self._register_user("refreshtoken", "secret123")
+        refresh_token = self._register_user("refep_refreshtoken", "secret123")
         client = TestClient(app)
 
         # First refresh
@@ -140,7 +140,7 @@ class TestRefresh:
         client.post(
             "/api/auth/register",
             json={
-                "username": "loginrefuser",
+                "username": "refep_loginrefuser",
                 "email": "loginref@example.com",
                 "password": "secret123",
                 "date_of_birth": "2000-01-01",
@@ -149,7 +149,7 @@ class TestRefresh:
         # Login to get a refresh token
         login_resp = client.post(
             "/api/auth/login",
-            data={"username": "loginrefuser", "password": "secret123"},
+            data={"username": "refep_loginrefuser", "password": "secret123"},
         )
         assert login_resp.status_code == 200
         refresh_token = login_resp.json()["refresh_token"]
@@ -166,7 +166,7 @@ class TestRefresh:
         """The access token returned by /refresh is a valid JWT."""
         from jose import jwt
 
-        refresh_token = self._register_user("jwttoken", "secret123")
+        refresh_token = self._register_user("refep_jwttoken", "secret123")
         client = TestClient(app)
         resp = client.post(
             "/api/auth/refresh",
@@ -190,7 +190,7 @@ class TestRefresh:
 
     def test_refresh_expires_minutes_in_response(self):
         """The response includes access_token_expires_minutes."""
-        refresh_token = self._register_user("expiretest", "secret123")
+        refresh_token = self._register_user("refep_expiretest", "secret123")
         client = TestClient(app)
         resp = client.post(
             "/api/auth/refresh",