You could make a new variable that indicates wether the door is open or not and then do the changes needed to close or open the door.
It would look something like this:
local garageDoor = script.Parent
local clickDetector = script.Parent.ClickDetector
local DEBOUNCE = false
local OPEN = false
local amount = 1
clickDetector.MouseClick:Connect(function()
if not DEBOUNCE then
DEBOUNCE = true
if not OPEN then
--Door open
OPEN = true
garageDoor.Size = Vector3.new(1, 2, 16)
garageDoor.Position = Vector3.new(14.5, 10, -12)
else
--Door close
OPEN = false
garageDoor.Size = Vector3.new(1, 10, 16)
garageDoor.Position = Vector3.new(14.5, 6, -12)
end
task.wait(amount)
DEBOUNCE = false
end
end)