Ive had a post liked by a deleted user. The board has always been XF, so not an import, and I only have 2 deleted users that immediately spring to mind, but these were deleted a long time ago, so im a bit puzzled as to why or how this happened
To be sure, we'd need to see the ID of the user attached to that particular like (in the xf_liked_content table). It could be something like merging users (though likes should be brought across if possible).
How did you identify that there was just one like? If you just browsed the table directly, you may have missed the other one as they won't necessarily be in order. You likely need to search by the post ID.
There is a unique constraint in place in MySQL that physically prevents the same post twice; the double news feed could be from a like/unlike.
If you did the like/unlike, that would rebuild the like cache which is reading the data fresh from the DB.