1, 5, 5, 9, 7, 11, 11, 15, 12, 17

The given sequence is a combination of two series:

I. 1, 5, 7, 11, 12 and II. 5, 9, 11, 15, 17

The pattern in both I and II is + 4, + 2, + 4, + 2. So, 12 is wrong and must be replaced by (11 + 2) i.e. 13.