I've tried to add an extra email address through "manage accounts". This seems to work, but I can't sign in using the new account details. Also the new account fails to display on the "manage accounts" screen.
If I try to add the new address again, the address appears to be taken.
What are the email addresses you are having trouble adding or which say they are already added? Pop the details across to me via PM (purple envelope, top right hand corner) and I will investigate further for you.