[fix] Add the same event listener only once

Prevent `WebSocket.prototype.addEventListener()` from adding the same
event listener multiple times.

lib/event-target.js

@@ -178,15 +178,25 @@ const EventTarget = {
    * Register an event listener.
    *
    * @param {String} type A string representing the event type to listen for
-   * @param {(Function|Object)} listener The listener to add
+   * @param {(Function|Object)} handler The listener to add
    * @param {Object} [options] An options object specifies characteristics about
    *     the event listener
    * @param {Boolean} [options.once=false] A `Boolean` indicating that the
    *     listener should be invoked at most once after being added. If `true`,
    *     the listener would be automatically removed when invoked.
    * @public
    */
-  addEventListener(type, listener, options = {}) {
+  addEventListener(type, handler, options = {}) {
+    for (const listener of this.listeners(type)) {
+      if (
+        !options[kForOnEventAttribute] &&
+        listener[kListener] === handler &&
+        !listener[kForOnEventAttribute]
+      ) {
+        return;
+      }
+    }
+
     let wrapper;
 
     if (type === 'message') {
@@ -196,7 +206,7 @@ const EventTarget = {
         });
 
         event[kTarget] = this;
-        callListener(listener, this, event);
+        callListener(handler, this, event);
       };
     } else if (type === 'close') {
       wrapper = function onClose(code, message) {
@@ -207,7 +217,7 @@ const EventTarget = {
         });
 
         event[kTarget] = this;
-        callListener(listener, this, event);
+        callListener(handler, this, event);
       };
     } else if (type === 'error') {
       wrapper = function onError(error) {
@@ -217,21 +227,21 @@ const EventTarget = {
         });
 
         event[kTarget] = this;
-        callListener(listener, this, event);
+        callListener(handler, this, event);
       };
     } else if (type === 'open') {
       wrapper = function onOpen() {
         const event = new Event('open');
 
         event[kTarget] = this;
-        callListener(listener, this, event);
+        callListener(handler, this, event);
       };
     } else {
       return;
     }
 
     wrapper[kForOnEventAttribute] = !!options[kForOnEventAttribute];
-    wrapper[kListener] = listener;
+    wrapper[kListener] = handler;
 
     if (options.once) {
       this.once(type, wrapper);

test/websocket.test.js

@@ -3257,10 +3257,13 @@ describe('WebSocket', () => {
       ws.addEventListener('foo', () => {});
       assert.strictEqual(ws.listenerCount('foo'), 0);
 
-      ws.addEventListener('open', () => {
+      function onOpen() {
         events.push('open');
         assert.strictEqual(ws.listenerCount('open'), 1);
-      });
+      }
+
+      ws.addEventListener('open', onOpen);
+      ws.addEventListener('open', onOpen);
 
       assert.strictEqual(ws.listenerCount('open'), 1);
 
@@ -3273,9 +3276,28 @@ describe('WebSocket', () => {
       };
 
       ws.addEventListener('message', listener, { once: true });
+      ws.addEventListener('message', listener);
 
       assert.strictEqual(ws.listenerCount('message'), 1);
 
+      ws.addEventListener('close', NOOP);
+      ws.onclose = NOOP;
+
+      let listeners = ws.listeners('close');
+
+      assert.strictEqual(listeners.length, 2);
+      assert.strictEqual(listeners[0][kListener], NOOP);
+      assert.strictEqual(listeners[1][kListener], NOOP);
+
+      ws.onerror = NOOP;
+      ws.addEventListener('error', NOOP);
+
+      listeners = ws.listeners('error');
+
+      assert.strictEqual(listeners.length, 2);
+      assert.strictEqual(listeners[0][kListener], NOOP);
+      assert.strictEqual(listeners[1][kListener], NOOP);
+
       ws.emit('open');
       ws.emit('message', EMPTY_BUFFER, false);
 
@@ -3353,21 +3375,6 @@ describe('WebSocket', () => {
       assert.strictEqual(ws.listenerCount('message'), 0);
       assert.strictEqual(ws.listenerCount('open'), 0);
 
-      // Multiple listeners.
-      ws.addEventListener('message', NOOP);
-      ws.addEventListener('message', NOOP);
-
-      assert.strictEqual(ws.listeners('message')[0][kListener], NOOP);
-      assert.strictEqual(ws.listeners('message')[1][kListener], NOOP);
-
-      ws.removeEventListener('message', NOOP);
-
-      assert.strictEqual(ws.listeners('message')[0][kListener], NOOP);
-
-      ws.removeEventListener('message', NOOP);
-
-      assert.strictEqual(ws.listenerCount('message'), 0);
-
       // Listeners not added with `websocket.addEventListener()`.
       ws.on('message', NOOP);