thermodynamics - Sonic Boom in Aircraft and Spacecraft

I would like to know why aircraft, and spacecraft produce a double sonic boom on breaking the sound barrier.

A while ago, I thought I got it, as there’s a start and finish point to every vehicle.

But is that it?

I already knew that condensation on wingtips and tail fins is caused by localised hypersonar and the pressure induced by the shockwave.

I researched a few articles and watched a few flights (not like I ever need an excuse! 😁), but still don’t understand why it isn’t a smooth lengthwise transition once the first shock has been achieved.

Any explanations most welcome, with thanks in advance!

Post scriptum; I don’t have 1000 rep, and so can’t tag this with max Q.

PPS Or where it generally happens either, apparently.