I get the DB error page when trying to rename a character to one that already exists on that server, or to move a character to a different server where a character with that name already exists.
At least before I left, there was a unique key/index on (tu_key, [strike]char_id[/strike]name, server_id) which I'm guessing is still there.
Possible solution
1. Alter the "edit character"/save character logic to check for existence on the key (if either value is changed)
2. Remove "unique" part of index
EDIT: it may have included global handle instead of tu_key. I don't remember at this point