Hi Gokhan,
In that case, you can prompt a message to the user by saying that you already login on another device. Please clear your session or log out etc to log in into this device. If he/she still wants to login in this phone then first validate his profile details according to the details which you have stored. Once he/she provide the details update his loginStatus in attribute and redirect him into game screen.
Let me know if you need any other help from our side.
Regards,
Himanshu Sharma