it is not necessarily a dual fil. bulb, some of the bikes made by Puch used a two bul taillight one bulb for high and one for low, some used the dual fil. bulb, but no the high should not be on if the low is burnt out, if it is a dual fil. it is possible that the bulb is in backwards, the wires are hooked up wrong so high is low and low is high, or if it is a two bulb system the low may have burnt out and the previous owner switched the bulbs so the light would be on.
Another solution may be that one of the brake light switches is malfunctioning and keeping the brighter light on all the time.