Event fired when clicked again?

I can't find this event on the ROBLOX wiki so i'll ask here.

What's the event that fires AFTER the player clicks a button the 2nd time? For example, if they click a button to make a GUI visible i want them to click it again to close it.

You'd use MouseButton1Down or MouseButton1Click. If you want it to close, use a variable.

open = false
vis = script.Parent.Parent -- Should lead to what you want to be Visible/Invisible
sp = script.Parent 

sp.MouseButton1Down:connect(function()
	if not open then
		open = true
		vis.Visible = true
	elseif open then
		open = false
		vis.Visible = false
	end
end)

There is no event to check if a player has clicked again. But there is a way you can do what you are talking about using variables:

open = false -- Gui is invisible by default
gui = script.Parent.Parent -- Assuming script.Parent.Parent is a Frame or something that has a visible property, you can change this of course
script.Parent.MouseButton1Down:connect(function() -- Assuming script.Parent is the button
	if open == false then -- If the gui isn't open
		gui.Visible = true -- Make the gui visible
		open = true -- Set the open variable to true
	else -- If it is open
		gui.Visible = false -- Make it invisible
		open = false -- Set the open variable to false
	end -- End the if statement
end) -- End the function

local gui = script.Parent.Parent:WaitForChild("Frame")
local button = script.Parent

button.MouseButton1Click:connect(function()
	gui.Visible = not gui.Visible
	-- Makes it the opposite of what it currently is.
	button.Text = (gui.Visible and "Close" or "Open")
	-- Ternary operation to shorten it. 
		--[[ 
		if gui is visible then 
			-- make the text Close
		--else
			-- make the text Open
		--]]		
end)