If you control multiple Undead Alchemists, the multiple replacement abilities will have no added effect. Combat damage dealt to a player by a Zombie you control will be replaced only once with cards being put into that player's graveyard.

Whenever a creature card is put into an opponent's graveyard from their library, the triggered ability of each Undead Alchemist you control will trigger. The first such ability to resolve will exile that creature card and create a Zombie token. Subsequent abilities won't exile the creature card, but each will create another Zombie token.