In Android, Why Is Facebook Signup Not Working If Facebook Installed In The Device Already?

I have tried to use Facebook in my application, this works fine on device that doesn't have Facebook app installed. But this fails to give the session while there is Facebook alrea

Solution 1:

I have answered for this type of (Hash for fb) issue here :

Facebook Login and Post To Wall works for first time only

Note: Fb gives issues bcz of not valid Hash, for that you have to generate a valid hash.