Teleport and duplicating models?

What you've done is only made it clone once when the script runs for the first time, if you want it to be cloned each time it's clicked put the cloning line in the on clicked function like so:

function onClicked(Part) -- Part is the player that clicked it...
	Case = game.ServerStorage.Case:Clone() -- Moved into the function
	Case.Parent = game.Workspace -- Moved into the function
	Case:SetPrimaryPartCFrame(CFrame.new(99, 2.5, 55.5))
	game.Players.LocalPlayer.Backpack.Case:Destroy() -- LocalPlayer is nil because this isn't in a player
end
script.Parent.ClickDetector.MouseClick:connect (onClicked)

Also, to avoid it erroring if the player doesn't have a backpack check it before removing it, It also looks like you're trying to make it look as if it's taking there bag, if you are use this:

function onClicked(player) -- Changed to player so it makes more sense
	if player.Backpack:FindFirstChild("Case") ~= nil then -- If they have a case
		Case = game.ServerStorage.Case:Clone() -- Moved into the function
		Case.Parent = game.Workspace -- Moved into the function
		Case:SetPrimaryPartCFrame(CFrame.new(99, 2.5, 55.5))
	end
end
script.Parent.ClickDetector.MouseClick:connect (onClicked)