Perhaps it recognises that you already have it and hence if you want to buy it for a friend you can get it for the 20% off that you would with a two-pack purchase?
I saw your screenshot and everything tells me its a glitch and the checkout wouldn't give it you for 20% off. no one else has it at 20% off unless you buy it as a 2 pack or as part of the Potato Sack.
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