You're correct in assuming this isn't a sequel. These are definitely different characters, though they could just as easily have been the characters from THE MISSION considering the thematic similarities. And Simon Yam is amazing in this movie, so full of smarm and (largely justified) hatred towards Nick Cheung's character in particular and just about everyone else in general.
Very mild spoiler below, but it won't ruin the movie, I promise!
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In typical Johnnie To fashion, one extended gunfight in a restauarnt (partly shown in the trailer at the movie's website) results in two major characters getting rather nasty wounds. One of the wounded men is driven to an "underground" medical clinic based in the doctor's apartment. Moments later, the other wounded guy, Simon, is brought to the same clinic and demands preferential treatment without realizing that the patient on the gurney he just pushed across the room is actually a guy he's wanted dead for ages, and that guy's pals are strategically hiding throughout the apartment, where another gunfight is about to explode while Simon's getting some stitchwork done to some very sensitive areas.
And then there's this hooker that the doctor was schtupping just before everyone came in...gotta keep your eye on that one!