A guy (Alfred) sent me an ECU where I replaced two burnt out IGBTs. The ECU controls a BMW M50, 6 cyl engine

The coils are wired in a non-standard way, they get fed voltage in pairs where each pair has it's own 7.5A fuse. So cyl 1+6 coils share a 7.5A fuse, etc.

What could cause the IGBTs to overheat?

Reasonable answers are excessive dwell or failure to remove fuses during an upgrade.

The coils connected to the outputs are also dead, hard to say which came first but the coils probably died from the IGBTs short circuiting. Shouldn't a 7.5A fuse blow and protect the coils from that?

Could "bad" coils cause this?

The coils that handle normal operation simply are not designed for twice the frequency, especially COPs that live on harsh temp cyl head. From double freq the coil lifetime can drop from 15-30 years to 1-10 months.

Should one use an even smaller fuse?

This doesn't seem to be a problem in most installations where you don't even have a separate fuse for the coils, like when using the stock M50 harness.

Information about the install is sketchy at best and I've requested full details and story. For one, the harness is custom made and the ground for the coils was connected to the chassis but had come loose. I don't know what came first.