Content should only get one rating from one user, but it may still receive additional ratings as long as they are from different users. If a user wants to change the rating they already submitted that is fine, but they should not rate it twice. That content can still get ratings from other people.
While this issue is related to reputation, not ratings, the same rules apply. They are more important regarding reputation because even if the prior database allowed it, when imported into XenForo, there are only two states: Liked and Not Liked. If pegasus is able to like the same post 3 times, it will say "Liked by pegasus, pegasus, and 1 other person."