How do make the touched function fire once?

You should use a debounce. Its an easy way to detect if a player already touched a part or not.

A debounce is a bool value we make to check if that value is true or not, we can do certain things if its true or false.

You can name it whatever you want.. however its known to be called debounce.. its pretty much like a "cooldown".

debounce = false --It will be false because the player didnt touch the part yet

script.Parent.Touched:Connect(function(hit)
	if debounce == false then debounce = true --It checks if its false, and if it is, it sets it to true.
            if hit.Parent.ClassName == "Model" and hit.Parent.Humanoid ~= nil then
            local character = hit.Parent
            character.HumanoidRootPart.CFrame = 
            CFrame.new(workspace["SecurityTP:A"].Position)
            local player = game.Players:GetPlayerFromCharacter(character)
            local clone = script.ScreenGui:Clone()
            clone.Parent = player.PlayerGui
            clone.Frame.Visible = true
            clone.Frame:TweenSizeAndPosition(UDim2.new(0.466, 0,0.185, 0),UDim2.new(0.266, 0,0.408, 0))
            wait(3)
            clone.Frame:TweenSizeAndPosition(UDim2.new(0,0,0,0),UDim2.new(0.5,0,0.408,0))
            wait(1)
            clone:Destroy()
	    debounce = false --After it has finished, you set it back to false and thats it.
		end
	end
end)

Hope this helped you

:)