You'll just get one. Z-crystals don't work like other items; once you get one in-game, you basically trigger a flag that says you're allowed to use it. You may notice that you can give the same Z-crystal to several different Pokémon, and it will disappear if you trade a Pokémon holding one to a game that doesn't have that crystal yet -- although it will appear to have been traded if that game does have it -- because it's not really "held" by a Pokémon, just a condition that you assign to it that takes up its item slot.