Ok, this makes more sense. Those duplicates are rows (records), not columns. I do not know of a way to eliminate duplicate records in ArcGIS Online automatically. You cannot delete records from an ArcGIS Online table. There are a couple of ways to eliminate your duplicates, one at a time. One is to do it on the map. You need to have editing enabled (from the settings), then on the map, start he editing and click on the feature to delete and select delete.
Another way, that I haven't tried is through the service URL, according to this post:
https://gis.stackexchange.com/a/403100