I think this depends on whether you are editing a FH tone on the unit, or one from "my tones". I.e. If you select a patch on the FH unit, edit it and then save, it will duplicate in "my tones". However if you start by selecting the tone from "my tones", edit and save, it will update not duplicate, however your tone on your FH unit will remain unchanged because you are editing the cloud copy only.
This is obviously a bit annoying so I only edit and update "my tones". Only when I'm happy will I then save each to the FH.