Why would anyone put a fuse in seies with another fuse, you might ask? I’ll tell you

[Enzo]

Found an inch and a quarter hunk of 1/4" bolt snapped into a fuse holder once. But at least it was gold plated bolt, like from a fancy car stereo.

[Axtman]

SWR amps also have the double fuse.

I bet 1/3 of the amps I work on have incorrect fuses. Many wrong fuses are auto fuses (32v).

[SoulFetish]

Come on guys.. we have all done it. Take a set break 3/4s through the night, flip the standby switch, fuse blows, you got one spare but it blows too! You take the power tubes out, wrap them in a bar towel and toss them lightly on a carpeted surface. Put them back in, foil around the fuse..... flip the standby and play the last set. Got me through the night a couple of times back in the day.

I'll say this, I've done enough dumb things at this point to know that being lucky is no substitute for informed.
Eventually, your luck runs out.