how to prevent the childremove function on Remotevent(Server Script) to stack?

Hello. You have to use :Disconnect(). Try this:

local Debris = game:GetService("Debris")

game.ReplicatedStorage.Arm.OnServerEvent:Connect(function(Player, Arg)
	if Arg == "Yeet" then
		Print("Faster")

		local a = Instance.new("Part", Player.Character)
		a.Name = "Sped"

		Debris:AddItem(a, 5)

		Player.Character.Humanoid.WalkSpeed = 32

		local removed

		local function onChildRemoved()
			if Removed.Name == "Sped" then
				Print("Slowed")
				Player.Character.Humanoid.WalkSpeed = Player.Character.Humanoid.WalkSpeed - 16
				removed:Disconnect()
			end
		end

		removed = Player.Character.ChildRemoved:Connect(onChildRemoved)
	end
end)

The script will disconnect the event when the child named "Sped" is removed. Please accept and upvote this answer if it helped.