In my application with a function to authenticate the user to use. I cann't twice log in so I log out and then log back in and face many problems in other functions.

So I can authenticate the user without having to log out or I may get passwords (encrypted password) to compare to enter user's password ??? Please help me. Thank you!!

Sorry for my english is not good. I use Google translate.

Kirito,

I will do my best to help you, but I am somewhat unclear of your problem.

You can only have 1 logged in user at a time on a mobile device, so when you call login again you are being returned an error in the library.  To turn on library debugging for Android you can select that option over here:  

http://devcenter.kinvey.com/android/guides/troubleshooting

If you need to login a second user, make an asynchronous call to logout, and in the success call back call login for the user that you are trying to login.

Please let me know if you have any other questions,

Thanks,