add test to verify duplicate controller are not registered

test/fixtures/controllers/hello_controller.ts

@@ -0,0 +1,15 @@
+// hello_controller.ts
+import { Controller } from "stimulus";
+
+export default class extends Controller {
+  static targets = ["name", "output"];
+
+  declare readonly nameTarget: HTMLInputElement;
+  declare readonly outputTarget: HTMLElement;
+
+  greet() {
+    this.outputTarget.textContent =
+      `Hello, ${this.nameTarget.value}!`
+  }
+
+}

test/manifest_test.rb

@@ -9,6 +9,9 @@ class Stimulus::Manifest::Test < ActiveSupport::TestCase
     assert_includes manifest, 'import HelloController from "./hello_controller"'
     assert_includes manifest, 'application.register("hello", HelloController)'
 
+    assert_equal 1, manifest.scan('import HelloController from "./hello_controller"').length
+    assert_equal 1, manifest.scan('application.register("hello", HelloController').length
+
     # CoffeeScript controller
     assert_includes manifest, 'import CoffeeController from "./coffee_controller"'
     assert_includes manifest, 'application.register("coffee", CoffeeController)'