Can you help me sign into my Twitter account via smartphone?

I’m using an Android OS Optimus V phone. I enter my Twitter account perfectly fine, and something pops up saying, “This account has already been added.” I’m thinking well that’s great! Hah but I can’t log in. I’ve already clicked the “sign in” button. Not the “sign up”. I’ve “cleared” data on settings>applications already. Can you site a source/link to any help? Thanks in advance.