It's tag only so if you tag again because lets say you were unsure you tagged, the second tag is disregarded, it's the same idea if I ride a bus but have to transfer I tag again but I don't pay again. It's just to verify I've tagged once since getting on. Can't say I've ever seen transit police but could be the buses I ride, and their's a fine for not being able to show you paid, but buses and muni is just tag as you get on. So theirs a risk of course.
Usually the reader has a local log of the last few cards read. If it detects the same card again within a certain time window it assumes it either read it twice - or two people are sharing a card - and ignores the second read
