How to add a one time stat change?

To prevent multiple people from clicking/touching/activating the same object multiple times, you can make use of a debounce table. Since the first parameter of MouseClick is the player instance that clicked the part then you don't really have to use :FindFirstChild() on the Players service.

local Debounce = {}

script.Parent.ClickDetector.MouseClick:Connect(function(player)
	--Return immediately if the player has clicked the block.
	if Debounce[player] then return end
	
	--If they haven't clicked the block then put them on the table.
	Debounce[player] = true
	local stat = player:WaitForChild("leaderstats")["Kirbies Found"]
	stat.Value = stat.Value + 1
end)

Please accept this answer if it helps.