Double possessive (of + mine etc)
Incorrect : He is a friend of me.
Correct . He is a friend of mine.
Incorrect : A friend of you told me the news.
Correct : A friend of yours told me the news.
Note : When we want to emphasize the person who possesses rather than the thing which s/he possesses, we usually use the double possessive (of + mine/yours etc.). Here the general rule (that a preposition is followed by an object form of noun or pronoun) is violated. "A friend of yours' actually means “One of your friends.”