The magician displays a deck of cards. The cards are different from a normal deck, because all the backs are different. Magician removes two cards from the deck placing them face up on the table. Two spectators introduce the cards into the deck at positions they select.
The magician removes the face up cards and the cards just above them. The pairs are found to be a perfect match for value and colour. For example if the face up card was five of Clubs, the face down card is five of Spades. Then he turns both the cards in each pair face down and the backs match perfectly, although all the other cards have different backs with no other duplicates.
And for a final climax, the rest of the deck is turned face up and ribbon spread on the table. All the cards are found to be identical, and different to the two pairs.
Supplied complete with the very special deck, and instructions, the trick almost works itself.
Deck is as shown in picture, video attached for effect reference only