Our group of players uses a rules variant that solves the problem of the dealing player having an advantage when slapping pairs. It requires all players to make an adjustment to the basic technique of laying cards onto the table.
To lay a card face up, grip the card on the edge closest to the pile that all players are dealing into, with your thumb on top of the card and your index finger below the card. Rotate the card so that it is face up by scraping the edge of the card that is closest to you along the surface of the rest of your deck. Then place the card onto the pile. This can and should be done very quickly, because the longer it takes you to execute this technique, the longer the other people playing have a chance to view your card while you remain in ignorance.
Additionally, slapping is done by the other, non-dealing, hand. No exceptions. If you slap with the same hand that dealt the card, the next person to slap gets the pile (if it's a valid pair). With this rule it is to your advantage to use your non-dominant hand to deal cards and your dominant hand to slap.
To deal with people "slapping in"to a game incorrectly, our group generalizes the rule that penalizes players already in-game that slap incorrectly. Any player who slaps incorrectly owes 2 cards to the pile. If that player is already in the game, they pay their debt immediately and play continues. If that player is not already in the game, then they pay that debt as soon as they actually enter the game. Usually, if a wannabe-player accumulates a large debt as a result of incorrect slapping, they'll voluntarily give up and leave the table.