Every time I put an alternator belt on my Doblo, it breaks about half hour later. Anyone got any suggestions as to why?

Does it just drive the alternator?
Something it is driving is seizing.
Spinning the alternator by hand may show it to be free, but under load it may well seize.
If a diesel, it may well have a freewheeling pulley. These seize and cause vibrations through the alternator. Then the alternator body cracks and the rotor can't rotate properly.

Get a very good look all around the alternator body, you'll probably find it cracked, so you'll need a new one.

Another thought, now I'm more awake, the tensioner could be seizing. This would show polishing/burning to the back face of the belt.
Tensioner could also be coming loose, so pushing at an angle.
Anything else driven, such as some water pumps, aircon, power steering, etc., can cause such issues.

If it is hydraulic power steering, with the pump driven from this belt, when the fluid is cold, first thing in the morning this time of year, applying full lock immediately or quickly can demand high fluid pressures which stresses the belts. (The first Rover 820, mid eighties, drove its steering pump from a camshaft, with a toothed belt. Many an engine destroyed on a cold morning as the owner reversed out of their drive, applied full lock, and stopped the inlet camshaft from turning!)

Thanks for your input. Finally resolved it - the tensioner was knackered