This is crazy already, we reported this time and time again. Yahoo Search is not always listening to a simple 301 redirect. We reported this in September and the issue continued to linger on month to month. In fact, many sites are suffering from good rankings in Yahoo, because of this canonical domain issue.
A new thread at WebmasterWorld shows the frustration. Two very senior members discuss the issue, saying they both have 301 redirects from the non www version to the www version. Because Yahoo indexed both, the site will likely be delisted from the index, according to one senior member.
Moderator jdMorgan, is the one having the issue now. He explained:
I just found out that Yahoo! has indexed a non-canonical version of one of my domains, despite the fact that this non-canonical domain has been 301-redirected to the canonical domain since inception.
So, despite a 301 redirect from example.com to www.example.com that actually predates the DNS going live ten years ago, Slurp has "inferred" this non-www domain and is now showing it and linking to it in their search result.
One member spoke to a Yahoo rep at a recent conference and the Yahoo rep told him he would look into it. I guess, he has not had the time yet.
Forum discussion at WebmasterWorld.
Update: This seems to be being resolved, see the update here.