Question: Is the first one redundant and proper, or is it redundant and not necessarily correct?
(1) He is a friend of Doug’s.
(2) He is a friend of Doug.
This is a grammatical issue I am curious about, as I have always used “of Doug’s”, not “of Doug” in such sentences. Your question has prompted me to do some more research.
Swan’s Practical English Usage, 3rd ed. does not address the controversy but does give several ‘double possessive’ sentences, such as She’s a friend of my father’s so presumably he thinks double possessives are okay, at least in some contexts.
Grammar Girl provides a lengthy discussion of double possessives, and provides useful distinctions that explain when they are correct, when and how they could be avoided, and when they are a mistake. People may question her authority, but she does provide a useful breakdown and cites authoritative references.
Richard Nordquist, over at about.com’s grammar page, comes down on the side of double possessives being correct, and gives several examples of their use in literature (Bronte, etc.) and an interesting summary of the history of this debate among grammarians.
In The Oxford Dictionary of American Usage and Style, Bryon Garner surveys the pro and con arguments, and concludes that double possessives are fine except in awkward sentences like Many friends of Mr. Smith’s, which should be reworded to Many of Mr. Smith’s friends.
Having read all this, I have not changed my mind: He is a friend of Doug’s is grammatically correct, and He is a friend of Doug is not. If He is a friend of mine is correct, and He is a friend of me is incorrect (which I definitely believe to be the case), then the same rules should apply to proper nouns to show possession in this construction.
I hope this at least gives you some food for thought.