Actually, this tactic has worked quite well for me. More commonly, it's a case of person A asking me to do X and then person B asking me to do Y not realizing

Actually, this tactic has worked quite well for me. More commonly, it's a case of person A asking me to do X and then person B asking me to do Y not realizing that I'm working on X already. When this happens, I usually say "I'm already working on X. Are you saying I should tell A I need to do Y first?" This puts them in a double-bind: do they tell you to work on Y and come off looking like an asshole to person A? If it truly is important (or they are assholes), they'll say yes. Then it's a simple matter of telling person A that you'll be behind on their project and letting politics take its course. If project X is really important to person A, then they'll go talk to person Y. Either way you're in the clear. Once you've gotten another person involved, it's much more difficult for someone to gaslight you in the manner you speak of.