Voldemort wanted to kill Harry with AK when he was 1 year old, which was understandable since he could not predict that that spell would rebound at him.
After acquiring a new body and thinking he has the same protection as Lily, he tried killing Harry with AK again before the final duel. Again, understandable, since he thought the protection would help him as well.
But when the second AK rebound, rendering him unconscious, that should have at least raised some alarms that something is wrong with the spell's effects on HP. He saw that AK didn't have an effect on the boy the second time.
So why did he keep using AK on him in the final duel?
I hope that there is something more to it than "third time's the charm" or "if I just keep pounding him with AK, it will get him eventually".