I am able to give the defend order multiple times to the same army on the same hex.
Also I was able to give a defend order to an army on a hex NOT marked in orange (so should have been invalid). Screenshot attached. 2 hexes that I occupy are marked in orange, the selected hex (blue) I do not occupy but was able to issue a defend order.
Also I was able to give a defend order to an army on a hex NOT marked in orange (so should have been invalid). Screenshot attached. 2 hexes that I occupy are marked in orange, the selected hex (blue) I do not occupy but was able to issue a defend order.