Indefinite pronouns refer to people or things without saying exactly who or what they are. "Quelqu'un/quelqu'un d'autre" are part of the indefinite pronouns.
How to express someone and someone else in French
Quelqu'un = someone
In affirmative sentences, quelqu'un means 'someone'/'somebody'
Quelqu'un (in questions) = anyone/someone
In questions, quelqu'un can mean 'anyone'/'anybody' or 'someone'/'somebody'.
ATTENTION: n'importe qui, which also means 'anybody', would not be used in questions: it literally means any body, 'doesn't matter who' any.
See N'importe qui = Anyone (French Indefinite Pronouns)
Quelqu'un d'autre = someone else
See also the related lesson: Autre chose/quelque chose d'autre = Something else (French Indefinite Pronouns)
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