I MUST throw my hat into the ring!!!!!!
A Double contact bulb has one of the contacts that operates a lesser illumination circuit - THE TAILLIGHT and the other contact operates a DIFFERENT CIRCUIT in the bulb which is a brighter output - STOP LIGHT! (and Turn Signals in USA Cars)
There is no way a single contact bulb can accept two seperate inputs and distinguish between them to operate seperately/differently.
If, you do indeed have single contact bulbs in a double contact socket then the bulb would be very bright as a taillight and would NOT get any brighter when the brakes were applied since there would be only one circuit/brightness for the bulb to respond to.