EXPRESSO NEWS!
JUST TESTED IT ON TRAINING MODE, MF'S e ability (Let it Rain) don't do multiple procs of Spellthief's Edge, just as no other dmg over time ability is capable of doing it since so many patches ago (RIP NASUS SUP). I would understand if you meant that the slow from the ability set you up for the autos and double-taps to proc more of the Spellthief's Edge's stacks, but even if you hit 5 players on one e, you'll still proc Speltheif's just once