You have to set both profiles (one you want to duplicate and the new one) in ATS/ETS2 NOT to sync. with Steam Cloud.
Then they should appear in C:\Users\Name\Documents\American Truck Simulator (Euro Truck Simulator 2)\Profiles.
If they are set to sync. with steam cloud they won`t be there and you won`t be able to duplicate profile.
It worked for me (I was duplicating ETS2 profile)