NPCs not being spawned when quickly moving in and out of the range

[derklaro]

(Moved from CloudNetService/CloudNet-v3#307)

Description of the bug:

In Minecraft, players are not being spawned when they are not in the tablist. When a player moves in, directly out and again into the range where NPCs will be spawned, they will not be spawned because they are not added to the tablist.

Expected behavior:

Before addinf the players twice to the tablist and removing them too early, they should not be added twice and the timer for removal should be restarted.