How do I make a tool stop giving to players if they already have one?

I tried to make some edits to your script, for more details check below.

--Edited script, be sure to move this script out of the tool and move it into the block you are going to hit, also move the tool to the server storage.
--If you don't want to use these changes (^) then just add line 12 and 13 (in my script) to line 21 (in your script)

local debounce = false

script.Parent.Touched:Connect(function(part) --something touched the part
local human = part.Parent:FindFirstChild("Humanoid")  --tries to find humanoid through the block that touched the part
	if human ~= nil and debounce == false then --if debounce is false, and we find the humanoid
		debounce = true --turns on debounce
		local player = game.Players:GetPlayerFromCharacter(human.Parent) --finds the player through the humanoid's parent, which is the character
		if player ~= nil then --if we find the player
			local tool = player.Backpack:FindFirstChild("YourToolName") --trying to find if the player has the tool
			if tool == nil then --sees if the player does not have the tool
				game.ServerStorage.YourToolName:Clone().Parent = player.Backpack --gets the tool from server storage and gives it to the player's backpack
				wait(2)
				debounce = false --turns debounce back to false
			end
		end
	end
end)