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Double Jump!

This lets players double jump. The addon is made properly and even predicted ~oooo~. You'll get errors if you don't have Team Fortress 2 installed (it's free, come on.)

Configuration
The multijump_default_jumps console variable sets the amount extra times players can jump. This can be set in your server.cfg as so (e.g. 2 extra jumps):
multijump_default_jumps 2

Installing Workshop Addons on Servers
http://wiki.garrysmod.com/page/Server/Workshop

Boring Developer Stuff
PLAYER:SetMaxJumpLevel(amount of extra times player can jump)

Do that somewhen after PlayerInitialSpawn