Tapping your PRESTO card multiple times depends on the transit agency and the specific situation. For most systems like the TTC (Toronto), you tap only when you enter the station or board a bus/streetcar to validate your 2-hour transfer window. However, on the GO Train or UP Express, you MUST tap both "On" at the start and "Off" at the end of your trip so the system can calculate the correct distance-based fare; if you forget to tap off, you will be charged the maximum possible fare for that line. You should never tap your card twice at the same reader in rapid succession unless you are traveling with a group and the driver has manually adjusted the machine for "Group Fare" (available on some regional bus routes). If you tap twice at a subway turnstile, the second tap may either be ignored or, if your transfer window has expired, it will deduct a brand new fare. Always listen for the specific beep: one green light/one beep usually means success, while multiple beeps or a red light indicates an error or an invalid tap.