wow congrats on the 200k! as someone who likes getting highscores I'm really happy to see someone also went for it in my game!
The double attack occurs during a Mania which happens when you kill enough enemies it only has a short duration but can last longer if you kill more enemies during it, you can see the progress of the mania around the border of the battle box which I see now isnt very obvious. It also makes enemies spawn more and moving/attacking quicker. Originally I intend for it to fire an upgraded version of the attack but didnt have enough time to design 6 new attacks so it just additionally fires the attack of the next die face.