Android : Multiple application instance, when clicked on notification in status bar, android

I have developed an chat application, in final stages to release. I have little bug which am unable to resolve. When my user receives the notification of new message (Just like whatsapp) and if the user clicks on the notification , then new instance of application is opened. Due to this behavior what happens is the message sent by the user is show twice in the list view. How can I open only one instance of application. I have put android:launchMode="singleInstance" this in my android manifest but still it opens same application twice when clicked on the notification. How can i stop this behavior.