|Definition - A noun phrase that shows possession by using both an s-genitive and an of-genitive.|
Example - He is a friend of my brother's.
Oxford English Dictionary - The term's first citation is from 1898:
"The pleonastic genitive, as in he is a friend of my brother's, is generally partitive = ‘one of the friends of my brother’."
(H. Sweet New Eng. Gram. II. 54)