Can multiple 40k psykers cast the same spell in the same phase?
So I've found out where the rule is in the book. The rule is found under the Matched Play Mission Rules, found at the bottom of page 215. 

Psychic Focus


  With the exception of Smite, each psychic power can be attempted only once per turn, rather than once per psyker per turn.


So this only applies to Matched Play, involving point limits and restrictions for organized events or tournament play. If you're playing Open or Narrative play matches, this does not apply.