Yes, SP means "Short Print" I'm not quite sure what you mean by showing two listings for a single card with two values, one with an SP designation and the other without. You might have to send me an example of a card listed this way and I can look further into it.
It's been awhile since I've opened up a copy of Beckett, but I do remember they used to list 2 prices, one being a high price for the card, the other a low price. The SP designation would be for both prices for that single card.
Topps recently has been issuing as a gimmick, two cards under a single number. One is the regular issued card of a current star player, and the other a Short Print variation Legend and both cards share the same card number. I'm not sure if this is what you are referring to.
If you could send me an example of a card that is listed this way, I might be able to clear it up for you. -Steven KeyMan
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