Hi...
I had the same problem. The system insisted that my Apple ID - which is, of course, also my email address - was already being used by an account!
There was no way around the problem.
Oddly - you may think - I could still Sign in to iTunes and manage my account. I could change the password, and do whatever I wished. That always worked.
But as soon as I tried to Sign in to Discussions (or Apple Support Communities as they like to call it) the system would accept my Apple ID and my password, demand I choose what they call a Username (they really mean a display name) but then immediately display an error message saying my Apple ID was already being used by an account, and I was stuffed.
It made no difference re-setting my cookies (or anything else) in my browser.
After I had taken two technical-help guys through this rigmarole I was assured:
it is a bug which is impossible (for them) to fix.
The answer, my friend, is to create another Apple ID!
It would appear that Apple use a distributed, relational database, and that the one associated with Apple Support Communities is corrupted, and they do not know how/yet want to sort it out. The corruption may well have occurred when Apple introduced Usernames? The database associated with iTunes etc appears to be OK.
And another thing, the web form employed in creating a new Apple ID is very flakey. You may have to try umpteen times before sucessfully navigating to its end. This flakeyness may also be assoicated with the damaged Apple ID database?
It is all a bit of a shambles, sorry.
