Warning! Username already exists

Adding another login to my list and I keep getting a pop up "Warning, The provided username already exists for this site, please use a different username and try again."

I found the other entry that is the 'conflict', but the URL is completely different.  I deleted the entry that was 'conflicting' and was able to add in my new one.  However, when I try to add in the other entry back in, it gives me the same warning again.  Any idea's how to get around this?   I'm not sure why it thinks it's a duplicate entry, completely different URL's and Titles in the entry... (the username is my e-mail)