Movement:Double Jump
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The double jump is a name given to a quick series of two jumps. The second jump must be triggered within 400ms of the previous jump's execution. If, at the beginning of a regular jump, the player's Z velocity is x units per second, then the player's Z velocity should be 2x per second when initiating a successful double jump.
Please note that doubling the Z velocity will not double the jump height, as gravity affects the calculations.
If you really care about why this is: Velocity Versus Height.