When ratings exist in both ResearchPoint and The Raiser's Edge for a given record, syncing the record may cause duplicate ratings to be added to the record in The Raiser's Edge. This article details why this occurs and how to resolve the duplicates.
When ratings are manually added or imported into The Raiser's Edge that also exist in ResearchPoint they are not connected. So the sync from ResearchPoint doesn't recognize them as being the same because they were not added by the sync. This causes the sync to re-add them as new ratings.