Fairy Tail's Aquarius can still materialize? In the Alvarez empire arc, she is shown to have materialized in spirit form once again to help out Lucy. 
But we all know the key that belonged to Lucy is broken. 

What does this mean? are there 2 keys? 
I'm happy she's back though
The answer was revealed in the following issue


   The key to summon Aquarius has been reborn somewhere in the world and the gate was opened by the Celestial King so that Aquarius could deliver a message to Brandish and Lucy