"I have already finished lunch."
I wondered if that was because it includes 已经, but the 已经。。了 pattern seems to be standard usage.
Is this another bad Duolingo question?
Chinese has a simple rule that states that you can remove all the unecessary stuff. 已经 and 完 already imply something has been done and finished, you are free to add 了 or not, but it is completely unecessary. Put it or leave it, both ways are right.