Here's more information on the first 가 which is distinct from the others.
가 in 누군가 is part of the -ㄴ/인가 suffix which creates a sense of "some" as an indefinite reference.
누구 by itself means "who" in a question, or "someone" in a statement.
누군가 (shortened from 누구인가) strengthens the sense of wondering or the indefiniteness since -인가 is the question form of -이다 which means "is"("copula", as in "AA is BB").
See these other words created similarly by attaching -ㄴ/인가 to a pronoun.
- 무엇(what, something) => 무엇인가 / 무언가 / 뭔가 (something).
- 어디(where, somewhere) => 어디인가 / 어딘가 / 어디선가 (somewhere).
- 언제(when, sometime) => 언제인가 / 언젠가 (someday, sometime, one day).
- 그것은 사람인가 아니면 귀신인가?: Is it a man or is it a spirit? (-ㄴ/인가? as a interrogative sentence ending")
- 누군가(가) 말했다: Someone said.
- 무언가 찾는 듯한 기색: An air of searching for something.
- 어딘가에 있을 이상향: A Utopia that must exist somewhere.
- 언젠가는 밝혀야 할 사실: A fact that I must reveal someday.