Fix LibEvLoop to ignore double IO streams

Author: kelunik

src/LibEvLoop.php

@@ -35,6 +35,10 @@ public function __construct()
      */
     public function addReadStream($stream, callable $listener)
     {
+        if (isset($this->readEvents[(int) $stream])) {
+            return;
+        }
+
         $callback = function () use ($stream, $listener) {
             call_user_func($listener, $stream, $this);
         };
@@ -50,6 +54,10 @@ public function addReadStream($stream, callable $listener)
      */
     public function addWriteStream($stream, callable $listener)
     {
+        if (isset($this->writeEvents[(int) $stream])) {
+            return;
+        }
+
         $callback = function () use ($stream, $listener) {
             call_user_func($listener, $stream, $this);
         };