This is a tutorial about how to implement a simple yet robust double-jump mechanic in your platforming games. The download below contains all of the code from the video, and was updated to include comments and remove a portion of code that was redundant. I encourage you to download and check it out if you have any questions!
Download *.capx: http://db.tt/RBj6u4Tl
The theory here can also be applied to engines such as Game Maker 8.1/Studio, Unity, & etc.