People who commented before me did not notice.
People who commented before I did, did not notice.
The usage of two 'did's did not sound good to me, so I was confused.
People who commented before me did not notice.
People who commented before I did, did not notice.
The usage of two 'did's did not sound good to me, so I was confused.
It seems that the second sentence has a tautology. Two consecutive 'did's make the phrase not very nice. So I would choose the first sentence as a correct one (since I did not see any mistakes in it):
People who commented before me did not notice.