Thanks for tip.
However, I'm still having issue deleting the duplicates.
SORT can't use ObjectID as sorting field. I had to ADD FIELD (ObjID), then assign it ObjectID value. After that, I was able to SORT using ObjID. However, when using DELETE IDENTICAL on the sorted feature layer, it deletes the rows from the sorted table, not the feature layer itself...
Is there an efficient way to apply the sorting to the original feature layer?