Do you know someone who is my friend?
A relative clause gives more information about something (a noun in the sentence); in this case that thing is denoted as "someone", an indefinate pronoun, and for this reason it is a very strange sentence which one would not expect to hear. But I wouldn't suggest that it is grammatically incorrect. It's just not the way we tend (usually) to express such a question.
Do you know one of my friends?
is far more likely.