Most of my house is lit by ceiling-mounted triple spotlights which use 40W
R50 bulbs. Recently, three bulbs in the same position in one particular
fitting have blown within a short time of being fitted. One lasted about
thirty minutes, one almost three hours, the third survived into the next
day. I've checked as much of the unit as I could without wholesale
disassembly and found nothing obviously wrong.

All three bulbs came from a DIY shed multi-pack. Though it seems unlikely,
is it possible that there was something faulty about a whole batch of them?
Failing that, what else could cause this problem?

Many thanks, and a happy and healthy new year to one and all.