That seems very dumb for me, hif someone have multiple copies of a popular card is normal to get more than one trade offert, and sometimes you want to get rid of more than one copy of the same card. This needs to be improved.
Maybe whit a warning or don´t letting you see a card hif you are using in on a trade, somethin like:
[Card_1] x5 Copies
(You proposed a trade using card_1)
(Now Neon Mob let you make more trades whit the same card, but displays instead)
[Card_1] x4 Copies