user warning: Duplicate entry '1-15' for key 1 query: INSERT INTO term_node (nid, tid) VALUES (15, 1)

When I try to create an event that is associated with two categories (one for protection, one to set the type of event, e.g. birthday or anniversary), I get this error:

user warning: Duplicate entry '1-15' for key 1 query: INSERT INTO term_node (nid, tid) VALUES (15, 1) in /.../includes/database.mysql.inc on line 172.

It happens every time. If I create an event with just the protection category (which is mandatory), no errors. Even if I try to edit the event and select, say, Birthday, I get this error.

However, the event is tagged with both categories...