The generally accepted scholarly opinion on Bible (in particular Pentuach (sp?) authrship is that Genisis and some other books are a combination of two authors: E who refers to God as Elohim, and J, who refers to him as Yahweh (it's J rather than Y because the original scholars were German, who tend to use a J for the English Y sound), and that these two different traditions were responsible for the duplicated stories.
Later textual analysis identified P, for Priest, responsible for Leviticus, the laws and the geneologies, and D, author of Deiteronomy, which is considered stylisticly distinct.
These distinct books were threaded together by R, for Redactor, who put the various stories together, and also added linking passages such as "Now it came to pass, after these things . . ."
There's a good summary
here on Straight Dope. though I stress I'm not a Bible scholar.