Student or Learner
Is the second "in" necessary here?
He was called on to fill in in an emergency.
Thanks a lot
Yes, "fill in" is the phrasal verb, and "in an emergency" says when. Because it's so inelegant, the person writing it might want to recast the sentence.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.