test: refusing fakes, failure-path tests, shared panic stubs

crates/application/src/watchlist/tests/add.rs

@@ -90,3 +90,21 @@ async fn test_add_to_watchlist_movie_not_found_by_id() {
 
     assert!(add::execute(&deps, cmd).await.is_err());
 }
+
+#[tokio::test]
+async fn adding_the_same_movie_to_a_watchlist_twice_keeps_a_single_entry_and_succeeds() {
+    let movies = InMemoryStore::new();
+    let movie = test_movie_with_year("Heat", 1995);
+    let movie_uuid = movie.id().value();
+    let user_id = uuid::Uuid::new_v4();
+    movies.upsert_movie(&movie).await.unwrap();
+    let deps = make_deps(Arc::clone(&movies), Arc::clone(&movies));
+
+    for _ in 0..2 {
+        add::execute(&deps, add_to_watchlist_command(user_id, movie_uuid))
+            .await
+            .expect("second add must not error");
+    }
+
+    assert_eq!(movies.watchlist_count(), 1);
+}