2, 1, 2, 4, 4, 5, 6, 7, 8, 8, 10, 11,?

The given sequence is a combination of three series :

I. 1st, 4th, 7th, 10th, 13th terms i.e. 2, 4, 6, 8, ?

II. 2nd, 5th, 8th, 11th terms i.e. 1, 4, 7, 10

III. 3rd, 6th, 9th, 12th terms i.e. 2, 5, 8, 11

Clearly, I consists of consecutive even numbers. So, the missing term is 10.