Why use a double fuse for a relay-controlled heater?
Just a guess.  If both sides are possibly driven from "hot" AC, then a fuse in each leg makes sense since each leg could get separately shorted to ground or neutral.

No, I haven't specifically seen this myself, and again, this is just a guess.  Keep in mind that consumer devices in particular are highly cost-optimized.  That means somebody thought about this and felt it was necessary to have two fuses, possibly for regulatory reasons.