In case some of you don't know what dual filament means. If you take out the bulb, most older ones push in and twist CC wise. Take a look inside, a single filament will have two little wires sticking up with a spring like wire between. A dual will have two, usually one is a little shorter. If one is burned out you may be able to see it broken. The bottom of a single has only one contact, dual has two. The idea is the tail light and stop are in a single bulb.
Swapping the bulbs is an easy test but if you have it out anyway for the small price I would get a couple replacements. Nice to have on hand anyway. This may not be the case with newer bulbs. I replaced a headlight bulb in my SUV and was shocked at not only how pricey but how much stuff you had to remove just to get the old one out.
Mark & Nancy
2004 Winnebago Vectra 40KD
Shep dog, R.I.P. Kenzie dog Toad 2015 Jeep Wrangler Willys Wheeler