Is it normal and/or accepted for an older post to be closed as a duplicate in favour of a newer one, or was this just a mistake on the part of those voting?
Yes, it is perfectly acceptable to mark an older post as a duplicate of a newer one. The duplicate banner is a road sign for future readers; it should point to the best answer(s) available, be they under the older or newer version of the question.
(The above is meant as an answer in general, not for the specific situation).
If you think that the direction of duplicate link should be reversed, flag for moderator attention and explain why.