So I got the double enchanting perk and went to go enchant Harkons Robes, and I couldn't! I assumed that something that only has 1 enchantment on it could be enchanted again. Doesn't that just make sense???

Sorry, once something is enchanted you can't re-enchant it even with the extra effect perk. You can only enchant something with two effects if it has not been enchanted.

This. But it's still pretty lame. One would naturally assume you'd be able to, but nope, Bethesda is literally the Thalmor.

Never understood that limitation. Keep extra pieces of armor that you want your end character to look like and once you get the enchants you want, go for it....but choose wisely or you will be hunting for it again! I found out the hard way. And death to the pointy ears!