There's a bug with double jump making it inconsistent and unreliable. Felt so bad that it was best to just turn it off.
One way I found to trigger this inconsistency was to jump with a single jump head first into a platform above, then land. When you next jump you can't do the seconde jump until you land gain.
Also just dropping off a ledge doesn't seem to allow the mid-air jump a double jump would normally allow.
Finally a quick suggestion, it'd be nice to be able to adjust the jump acceleration and float time in addition to the fall gravity. Default acceleration/float time felt a bit floaty for my tastes.
But overall a fun little tool to play with and get a feel for the type of platformer control I'd implement if I were to make such a game. So good job.