The bulbs are dual filament bulbs. You need to replace the affected bulbs. While you are doing so, take a look at the conductors both on the old bulb and down in the socket. If these are chared or discolored, you really should clean them up for best bulb life.
Also, before slapping the new bulbs down into the sockets, put a liberal shot of dielectric grease down in there. I was having problems blowing out my brake lights all the time (once every month). I took the time to disassemble the socket (pull all contacts and wires out - very easy to do) and clean everything up very well. Then put in the dielectric grease and I haven't had a problem since and it'* been close to a year after I did the repair job. While I was at it, I did the same thing to all of the bulbs across the back just to be sure that it wouldn't happen to one of them down the line.