Oh, this is a pretty funny one. Basically, back when I was in middle school, I really badly wanted to become a youtuber like Pewdiepie, but I had to come up with a cool name, a name which stood out. I wasn't able to find any and, at a certain point, I also remember opting for "TheCobra260" or something like that. That stuff was far too edgy and didn't reflect how I actually am so, for a period of time I just kept thinking.
Then, one day, I came up all of a sudden with the name "Doublekey". I don't know why precisely and how, but my brain just told me "Hey, what about Doublekey? It sounds cool, doesn't it?". I thought it was quite a creative name so, in the end, I decided to keep it. Later on, I also gave this name to a main character of mine (he was the protagonist of my Pokemon fan series. It's a long story, maybe one day I'll tell it). So, yeah, unfortunately there's not much of an origin story, I just kinda came up with it along the way and thought it sounded cool and mysterious at the same time. A youtuber named Doublekey who made animations and played games, to me, seemed quite a cool idea.