I had a customer with a Peugeot 406 HDI taxi a few years ago that was getting through loads of bulbs. As usual made loads of tests to check charging voltage etc. It always seems to be the diesels. I am suspicious that when the alternator unloads as the glowplug relay turns off there is a voltage surge before the regulator gets things back under control. Due to the manner of use as a taxi with frequent starts and a lot of night driving I think it makes them more vulnerable to the problem. I suggested starting the engine and waiting for a minute or two (while putting seatbelt on etc.)before turning the headlamps on, he later reported that this seemed to have improved the situation.
As Tony says H7s never seem to last very long anyway and vehicles with calcium batteries charge at a higher voltage.
Shaun.