fix (ui): select the same episode multiple times

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'