Why are some sources cited twice in a bibliography, once with DOI and once with arXiv?
I agree with Dave Clarke.  I think the primary purpose of a bibliography should be to offer the reader a way to read the cited material.  A version of the paper on the arXiv achieves this goal better than any other means I know.  However, it's also important that the cited material has been verified.  By citing the journal version of the paper, the author communicates that it has been.