Only add event listeners once

See #372

src/services/player.ts

@@ -441,14 +441,18 @@ export default class {
       return;
     }
 
+    if (this.voiceConnection.listeners('disconnect').length === 0) {
       this.voiceConnection.on('disconnect', this.onVoiceConnectionDisconnect.bind(this));
+    }
 
     if (!this.dispatcher) {
       return;
     }
 
+    if (this.dispatcher.listeners('speaking').length === 0) {
       this.dispatcher.on('speaking', this.onVoiceConnectionSpeaking.bind(this));
     }
+  }
 
   private onVoiceConnectionDisconnect(): void {
     this.disconnect(false);