Due to nature of Darth Vader's presence, there was no way for anyone to verify Vader's identity (talking about Force Presence, there was a thing called Force Stealth).
Did Empire ever use Darth Vader duplicates to do its tasks? Mere Vader's presence could have made a job done thanks to his terror personality. In case Vader was not available, it could be useful.
Or, did Rebels ever try that? Well, using Vader duplicates, big things could be accomplished like gathering intel etc.
Legends is welcome...