We have the same issue here. In fact, it's enough of an issue, we have been working on how to fix it locally to our company. Right now the only fix we have, is to delete the user by hand from the database, and then allow the sync to happen. This is also a problem for us when a person gets married and their name changes. LDAP just changes the record, but Moodle creates a who new record because it's a new name. Its a fun time. Sorry, not really any help other then to say, I don't know how to fix it with out altering the physical program.
Authentication
LDAP: using a primary key that isn't username?
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