If you are using Firebase Cloud Messaging to send notifications to your application, then it means you are able to create your own notification. Then all you have to do is set the proper FLAG for your intent:

public class MyFirebaseMessagingService extends FirebaseMessagingService {

@Override
public void onMessageReceived(RemoteMessage remoteMessage) {
    try {
        Intent notificationIntent = new Intent(context, MainActivity.class);
        //Intent.FLAG_ACTIVITY_NEW_TASK Launches a new instances of your Activity. 
        //So if there is already one in background, there will be two instances.
        //Intent.FLAG_ACTIVITY_CLEAR_TOP is what you want, it resumes the last
        //session if there is any, or launches a new instance if there is no instance in background
        notificationIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP);
        PendingIntent pendingIntent = PendingIntent.getActivity(context, requestID, notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT);
        //do the rest
    } catch (Exception e) {
        e.printStackTrace();
        return null;
    }

}

I solved this problem by changing launch mode of activity, so, in AndroidManifest, I set launchMode = singleTask for MainActivity.

I’m using FCM to send notifications to my app, but there is a problem. if my user is using the app and just press the home button( not back key) to exit the app and got a notification, then press on the notification, my app will reopen. if the user presses the back button, he/she exits from the reopened app and he/she will be directed to the last state of the app just before pressing the home button.(in the other word, the user should presses back button twice, once for exiting from reopened the app and another for exiting from the app in the background)

how can I load the last state of the app if it is in the background and reopen the app if it’s not, by taping on the notification?

EDIT :

I changed “onMessageReceived” to code bellow but nothing changed:

class MyFireBase() : FirebaseMessagingService(){

    override fun onNewToken(p0: String) {
        super.onNewToken(p0)
    }

    override fun onMessageReceived(p0: RemoteMessage) {
        try {

            val intent = Intent()
            intent.action = "ACTION_STRING_ACTIVITY"
            intent.putExtra("category", p0.data["category"])
            sendBroadcast(intent)


            val notificationIntent = Intent(baseContext, MainActivity::class.java)
            notificationIntent.flags = Intent.FLAG_ACTIVITY_CLEAR_TOP
             PendingIntent.getActivity(
                baseContext,
                1,
                notificationIntent,
                PendingIntent.FLAG_UPDATE_CURRENT
            )

            super.onMessageReceived(p0)
        } catch (e: Exception) {
            e.printStackTrace()
        }


    }

}