How can i check if my android app is already running to prevent double launch?

How can i make "hard exit" to prevent my app running on background?

It is not possible to have "double launch". If you application is running already, if you try to launch another instance, than you'll resume the first launch instance.

You can finish activity by adding .finish() in all scenarios when application can be in onPaused() sequence of the lifecycle or add finish() in onPaused()