rework test to better reproduce mentioned issue

Author: arturgawlik

lib/net.js

@@ -533,7 +533,10 @@ Socket.prototype._final = function(cb) {
     debug('_final: not yet connected');
     if (!this._finalizingOnConnect) {
       this._finalizingOnConnect = true;
-      this.once('connect', () => this._final(cb));
+      this.once('connect', () => {
+        this._final(cb);
+        this._finalizingOnConnect = false;
+      });
     }
     return;
   }
@@ -806,7 +809,10 @@ Socket.prototype.destroySoon = function() {
     this.destroy();
   else if (!this._destroyingOnFinish) {
     this._destroyingOnFinish = true;
-    this.once('finish', this.destroy);
+    this.once('finish', () => {
+      this.destroy();
+      this._destroyingOnFinish = false;
+    });
   }
 };
 

test/parallel/test-net-socket-not-duplicates-destroy-soon-listeners.js

@@ -9,10 +9,14 @@ const socket = new net.Socket();
 socket.on('error', () => {
   // noop
 });
-socket.connect({ host: addresses.INVALID_HOST, port: 1234 });
-socket.destroySoon();
-socket.destroySoon();
-const finishListenersCount = socket.listeners('finish').length;
-const connectListenersCount = socket.listeners('connect').length;
-assert.strictEqual(finishListenersCount, 1);
-assert.strictEqual(connectListenersCount, 1);
+const connectOptions = { host: addresses.INVALID_HOST, port: 1234 };
+
+socket.connect(connectOptions);
+socket.destroySoon(); // Adds "connect" and "finish" event listeners when socket has "writable" state
+socket.destroy(); // Makes imideditly socket again "writable"
+
+socket.connect(connectOptions);
+socket.destroySoon(); // Should not duplicate "connect" and "finish" event listeners
+
+assert.strictEqual(socket.listeners('finish').length, 1);
+assert.strictEqual(socket.listeners('connect').length, 1);