I thought it was not possible, but look at this user. Agricola, Android etc etc. All rated multiple times. How is that possible?
It is possible. You have to add the game multiple times to your collection, and you can rate each of them independently.
It makes sense if you connect the individual items to different versions. For example, i rate the original edition of Auf Achse at 4, and the recent edition at 7.5.
Note that for BGG ranking calculation, only the highest rating from each user is used.