If I have, both, a sticker and a FasTrak Flex transponder mounted inside my vehicle, will I be charged twice?

If you have, both, a sticker and a FasTrak Flex^® switchable transponder in your vehicle, only one will be read.
 
Some tolled bridges, lanes and roads in California offer discounted rates or free rides to carpoolers and/or high occupancy vehicles. When driving those facilities with two transponders, the appropriate toll will be collected based on the settings on the FasTrak Flex transponder, if a discount is applicable. Learn about carpooling with FasTrak Flex.

On The Toll Roads, it doesn’t matter which transponder is read for toll collection because discounted tolls are not offered and every vehicle must pay a toll.