Not familiar with that enlarger, but if it has a voltage regulator of some description, you may want to check the voltage at the lamp sockets to make sure it is not feeding them excess voltage.

If they are halogen (you probably know this already but it's worth repeating for anyone that doesn't) do not touch the lamps with your bare fingers: the oil from your skin will cause hot-spots that will cause the lamps to fail prematurely.

Thinking about it, it is possible each pair of lamps are wired in series, in which case only one of a pair may have actually died and that one of each pair is still OK.

My best guess is that Nick has hit the nail on the head and you will have a couple of years wait for another surprise...

Cheers, Bob.