Do you know that there is stencylforge? Just click view on the top of the screen. When you click that view button, click stencyforge. From there, click behaviors. If you don't see a double jump behavior, just search "double jump" in the search-box on the middle-top of the right side. Hope this helps. If you want an actual code (I mean logic). Please tell me. P.S.: you can also see how a double jump works by opening the behavior editor.
