Mapping multiple keys/buttons to a single input is permitted but only one of those keys/buttons may be used per stage

Without going into how dumb this rule is for not just flat out forbidding multi-binds, how is this checked? The keyboard runners I see now, it can be very hard to see what keys they are using.

Also is 6-2, 6-3 and 6-4 considered seperate "stages" for this rule?