diff --git a/.gitignore b/.gitignore
index bc353e8..06b3256 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,3 +4,4 @@ __pycache__/
 .venv/
 venv/
 .DS_Store
+.sqlite3
diff --git a/app/services/session_service.py b/app/services/session_service.py
index 5b6d450..733eb23 100644
--- a/app/services/session_service.py
+++ b/app/services/session_service.py
@@ -341,6 +341,21 @@ class SessionService:
             return self.list_selected_episodes(session_id)
 
         with self._connect() as conn:
+            existing_rows = conn.execute(
+                """
+                SELECT payload_json
+                FROM selected_episodes
+                WHERE session_id = ?
+                """,
+                (session_id,),
+            ).fetchall()
+            existing_episode_ids: set[str] = set()
+            for row in existing_rows:
+                payload = json.loads(row["payload_json"])
+                episode_id = str(payload.get("id") or "").strip()
+                if episode_id:
+                    existing_episode_ids.add(episode_id)
+
             current_max = conn.execute(
                 """
                 SELECT COALESCE(MAX(position), -1) AS max_position
@@ -352,6 +367,9 @@ class SessionService:
             next_position = int(current_max["max_position"]) + 1
 
             for item in items:
+                episode_id = str(item.get("id") or "").strip()
+                if episode_id and episode_id in existing_episode_ids:
+                    continue
                 conn.execute(
                     """
                     INSERT INTO selected_episodes (session_id, position, payload_json)
@@ -359,6 +377,8 @@ class SessionService:
                     """,
                     (session_id, next_position, json.dumps(item, ensure_ascii=True)),
                 )
+                if episode_id:
+                    existing_episode_ids.add(episode_id)
                 next_position += 1
 
         return self.list_selected_episodes(session_id)
diff --git a/data/session_state.sqlite3 b/data/session_state.sqlite3
index 3e6d2aa..f083fe3 100644
Binary files a/data/session_state.sqlite3 and b/data/session_state.sqlite3 differ
diff --git a/feature_tests_selected_episodes.sh b/feature_tests_selected_episodes.sh
index 5444e83..3085513 100755
--- a/feature_tests_selected_episodes.sh
+++ b/feature_tests_selected_episodes.sh
@@ -66,6 +66,24 @@ assert data["items"][1]["episode"]["title"] == "A Classic New York Character", "
 print("add/list validation passed")
 PY
 
+curl --fail --silent --show-error \
+  -X POST "${BASE_URL}/api/session/selected-episodes?session_id=${SESSION_ID}" \
+  -H "Content-Type: application/json" \
+  --data @"${TMP_DIR}/add_payload.json" \
+  -o "${TMP_DIR}/add_duplicate_response.json"
+
+python3 - "${TMP_DIR}/add_duplicate_response.json" <<'PY'
+import json
+import sys
+from pathlib import Path
+
+data = json.loads(Path(sys.argv[1]).read_text(encoding="utf-8"))
+items = data.get("items")
+assert isinstance(items, list), "items must be a list"
+assert len(items) == 2, "duplicate add should be skipped; still expected 2 selected episodes"
+print("duplicate skip validation passed")
+PY
+
 echo
 echo "== Feature test 2: reorder selected episodes =="
 cat > "${TMP_DIR}/reorder_payload.json" <<'JSON'