Here's a quick way to enable a Double Jump feature in your MPAGD platform games. It also includes a key debounce so that the Double Jump isn't initiated automatically by keeping your finger on the fire/jump button/key.
I've used a variable, J, that will store our current jump status, you could use a sprite variable like SETTINGA if you prefer. For the Double Jump feature there are five possible jump status:
0 No Jump active
1 1st jump has been initiated
2 A second jump is possible
3 A Second jump has been initiated
4 The player has landed after doing a jump.
By using an IF ELSE ENDIF on our fire key, we can ensure the right status is set which will the allow (or) not a second jump. This code will also ensure the fire key has been released before any type of jump (single or double) can take place.
Here it is in action, I've displayed the J Value at the top of the screen to help you see what was going on!