I've always had the thought that I can only watch a movie once.
What we've noticed is that we will start watching a movie we think we have never seen before, and about 15 minutes into it, realize we've seen it before... but we cannot predict the next few scenes, sometimes we do know the ending but the middle part is a blank. So we still enjoy it.
We joke that should we both get alzheimer's really bad, we can just buy 5 movies and watch them repeatedly to the end.
Yes it does worry me, but everything else seems ok...