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
Comments
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