the users won’t be confused as to why they need to add the same image twice if they want it to be the featured image and be in the gallery.
it is just a matter how you label that image-field and what description text you set, and that it is after the gallery-field.
also user don’t had to re-upload image, he should just “re-select” a already uploaded image. if you only show images uploaded to that post (for gallery and for featured image) it should be no problem and not confuse,
if gallery dont have 50+ images it will be also relative easy to use.
but that my humble opinion, if you dont think so i have no problem with that
