Regarding double fusing for IEC connectors:
Thin about the German Schuko plug, which is not polarised, so you don't know which pin is phase and which is neutral.
For a fault between the phase and neutral wiring in your device, a single fuse in the device will protect your wiring, regardless if t ends up to be in the neutral or phase line.
Now think about a fault to earth between any phase or neutral wire in your device and ground/earth. There are plenty of outlets without RCD in the world, so you can't assume an RCD is there which would trip. So you can a) make every wire and PCB trace in you device at least 0.75mm2 so that you know for sure the circuit breaker of the outlet, which should not exceed 16A (or 20A in the US, but polarised plugs there) will protect you from fire. Or b) put a fuse in both supply lines.
And regarding the dangers: Why should it be dangerous to use double fusing in a device? A blown fuse does not mean a device is safe to open and touch the wiring anyway, there could always be unfused parts of the wiring and components protected by a second fuse that is not obvious...
Its a different story of course if someone puts a separate breaker in the neutral line in a distribution box, that is a huge no-go: Neutral must never be disconnected without the corresponding phases in a 2- or 3-phase system, otherwise you end up with a floating neutral that takes on a voltage determined by the loads on the different phases, so overvoltage damage to connected devices is likely; in a 1-phase system this won't happen, but then turning off a breaker on a power panel and the corresponding circuit remaining hot is quite obscure and dangerous.