I would even go as far as to say that double hashing can actually make more collisions. If you get a collision with the first hash, you absolutely will get a collision with the second hash, but if you don't get a collision with the first hash, you still have a chance of getting a collision with the second hash.

say you start with X and Y.

hash(X) = X'
hash(Y) = Y'

hash(X') = X''
hash(Y') = Y''

if X' = Y' (collision with first hash), then X'' = Y'' (collision with second hash)
if X' != Y' (no collision with first hash), then X'' may = Y'' (possible collision with second hash)