PHP Error: Duplicate entry ... for key 1 query: INSERT INTO users_roles (uid, rid) VALUES...

I get a lot of PHP errors of the form:

Duplicate entry '537-16' for key 1 query: INSERT INTO users_roles (uid, rid) VALUES (537, 16) in includes/database.mysql.inc on line 172.

This is caused by the INSERT query in _ldapgroups_grant_role(). Though the SELECT query returns 0 rows, the INSERT query a couple of lines later still fails. I believe this happens if a user attempts to open multiple pages in different browser tabs at the same time.

Would it be possible to do a diff on the user's roles instead of removing them all and adding them all back? Seems like a lot of excess database work (two SELECTs, an INSERT, and a DELETE *per role*) where a single SELECT would cover the most common case (a user without different roles logs in).

I've inserted an IGNORE in the INSERT query to stop the PHP errors for now.