Damage script help?

Okay so I've made a damage script that when you touch a part you're losing 20 health.The bug isn't in the script but I want to make it so like that part damages the character only once. If I destroy the part it works but I don't intend to do it. Any suggestions?

x.Touched:connect(function(Part) 
    if Part.Parent:FindFirstChild("Humanoid") ~= nil and  Part.Parent:FindFirstChild("Humanoid") ~= Player.Character.Humanoid  and Part.Parent.Name ~= "Wall"  then
		Part.Parent.Humanoid.Health = Part.Parent.Humanoid.Health-20
		end
end)

First: Don't Do The Damage Like That, Use:

humanoid:TakeDamage(20)

Second, you should use hit:

script.Parent.Touched:connect(function(hit)
	local humanoid = hit.Parent:FindFirstChild("Humanoid")
	if humanoid then
		humanoid:TakeDamage(20)
	end
end)

Also, if you want it to make just 20 Damage, you should use script.Disabled:

script.Parent.Touched:connect(function(hit)
	local humanoid = hit.Parent:FindFirstChild("Humanoid")
	if humanoid then
		humanoid:TakeDamage(20)
		script.Disabled = true
		wait(2)
		script.Disabled = false
	end
end)

There, works. Also, If you copy my script, then you have to put it in the part.

You're Welcome,

SwaggyDuckie