QI use this follower to copy an unevolved follower with an evolve ability. If I happen to have the evolved version of the copied follower in my evolve deck, can I play the copied evolve ability?

AYes, but you would not gain access to the copied evolved follower's abilities.

When evolving, you would place the evolved version of the copied follower from your evolve deck on top of the unevolved Hamsa, Hallowed Fowl on your field and place the evolved version of Hamsa, Hallowed Fowl faceup on top of your evolve deck. However, the information copied by this follower's On Evolve ability takes precedence, so you would not gain access to the abilities of the evolved version of the copied follower.