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.
It’s a double genitive, described in the ‘Longman Student Grammar of Spoken and Written English’ as
. . . a special construction in which either the independent genitive or a possessive pronoun occurs in an of-phrase . . . [T]he main noun phrase typically begins with the indefinite article. In fact, the definite article does not normally combine with the double genitive . . . The meaning of the double genitive can sometimes be alternatively expressed by other constructions.
This is sometimes referred to as a "double genitive", though it's been suggested that it should in fact be considered a third form of genitive construction, along with the 's form and the bare of form. For this reason, "oblique genitive" has advantages as a term, since it doesn't contain double in the name.
It's interesting to note that there are different restrictions upon, and uses of, the oblique than the other forms.
In a question here, the correctness of using the Saxon ('s) genitive with inanimate objects was questioned. While the consensus was that you can certainly do both, my answer pointed out that some, like Fowler, have objected to it, and offered a possible explanation to the history of the objection.
Here though, we have a form that is more restricted, and perhaps this is really (or also) the reason for that other objection.
Consider that not only can we use the oblique genitive in this case, there are times where it is more natural:
*A friend of me visited.
A friend of mine visited.
It's also worth noting that the oblique genitive works better for some uses of the genitive, than others. Consider:
Mother's picture hung above the fire.
We could interpret this as either a picture that belonged to mother (possessive) or a picture that portrays mother (composition, or partitive). The meaning is ambiguous unless we can tell from context, or which is most likely, such as:
The King's picture hung above the fire.
Probably means a picture that portrays the King, because we tend to rarely encounter a picture owned by kings compared to pictures in which kings are the subject. (Though Oscar Wilde might perhaps point out, that the King is not a subject).
The two forms using of are clearer:
A picture of the king hung above the fire. (The picture shows the king).
A picture of the king's hung above the fire. (You should really hang pictures the king gave you somewhere better, though it's better than nailing it to a ceiling [long story]).
So here, the oblique genitive gives us the reading (or at the very least, leans toward it) of the possessive use of the genitive, rather than any other.
However, there are further restrictions:
?The picture of the king's hung above the fire.
*The picture of mother's hung above the fire.
*I met the friend of Doug's.
I met the friend of Doug's that everyone had been talking about.
The first seems doubtful to me, but perhaps okay. The second and third doesn't while the fourth which adds a restrictive modification, is okay.
So there's an anti-unique quality to the form, where it can only be used for a specific individual if item if that restriction comes from elsewhere. (Maybe the reason I'm not completely against the first is there's an implied unusual quality about the picture of the king's that partly excuses it).
An explanation that has been given is that of in the two forms actually serves a different purpose in each. The of in "The picture of the King's" is closer to the of in:
One of the King's pictures.
There are complications and difficulties beyond that, but what linguists have to say on it quickly gets beyond my dilettante interest. For those of us whose primary interest in grammar is to use it effectively, we can just consider that there are not (as often said), two forms of English genitive; that which uses 's or a genitive pronoun, and that which uses of. There are in fact three, and this "oblique genitive" is the third form.
I have always used the form
I am a friend of Doug's.
Therefore, this the analysis if the converse is acceptable.
Let us analyse this under diverse situations the following acceptable use.
Note that it would not sound fluent if we said
It is a concern of the state of New York Police Department's.
I am a friend of NYPD police officers' union leader's.
Therefore, to be fair and "logical", my analysis says it should be acceptable to say,
I am a friend of Doug.
Is my analysis flawed? Because I would still persist to use the form
I am a friend of Doug's.
Despite my reservations, I propose that it is "logical" and acceptable usage.
I am a friend of Doug.
"Correct" is beside the point here. Nothing says that language should be logical of free of redundancies.
Suffice it to say that double possessives are both common and colloquial in English. Search Google Books or the New York Times archives for countless examples. In the words of Bryan Garner: "The double possessive appears in good writing and typically causes no trouble. Occasionally, however, it can be improved [...]" (Garner's Modern American Usage, Possessives).
Both are perfectly fine then, depending on the context.
Language will always have little idiomatic quirks that cannot be explained by rules alone. If you don't have the ear for it, use literature and newspaper archives to ferret out the more natural alternative. For a pragmatic compromise between description and prescription, read "Making Peace in the Language Wars" in Garner's.