I apologize if this was already mentioned before but this might be faster a bit faster and consistent than just doing going again only to mess it up once again.

(Might work with forwards duping as well).

Basically, Jump, hit F3, wait 'til Tommy's landing animation to start then F3 again. Be sure to be at the very left of the marker otherwise you'll trigger the phonecall. ¤¤Note that the intention is only if you were to early too hit the mission marker¤¤ (although if you want to make it a personal way of duping go ahead).

E: Waiting until Tommy's landing animation is really tight, activate a bit earlier, messing (early) up isn't a bigger deal than the backwards/forward duping.