Harcore char killed by TWO Beheader Ataguchu

Just lost a hardcore character due to a double spawn Beheader Ataguchu. I checked other videos of encounters and it seems that he usually only spawns once. When I encountered him there were two spawned on top of each other. I pretty much got instantly annihilated before I could react after not dropping below 80% up to that point.

Being that it's a 'unique' mob I'm guessing this is a bug? Or are things like this supposed to happen occasionally.

Not overly upset about losing the character, but knowing stuff like this happens in this game makes me want to give it up. It's not a fun way to lose a char.

Beheader Ataguchu is a vaal area boss right?

As such they can be effected by the mods on the vaal area like contains 2 bosses