Why are 2 rows affected in my `INSERT … ON DUPLICATE KEY UPDATE`?

Question

I\'m doing an INSERT ... ON DUPLICATE KEY UPDATE for a PRIMARY KEY in the following table:

DESCRIBE users_interests;

Answer 1

From the manual:

With ON DUPLICATE KEY UPDATE, the affected-rows value per row is 1 if the row is inserted as a new row and 2 if an existing row is updated.

Answer 2

So you know whether you updated a row (duplicate key) or just inserted one: http://dev.mysql.com/doc/refman/5.0/en/insert-on-duplicate.html