git-fetch fetches blobs that are already in the local repository if no history is shared?

Hi all- 

There are two main git mirrors of the WebKit Subversion repository: <git://git.webkit.org/WebKit.git> and <https://github.com/WebKit/webkit>. These repositories have the exact same trees/blobs, but have entirely different commits due to the GitHub mirror using a custom --authors-prog with git-svn.

Tor Arne (CCed) noticed something interesting today:

If you clone one of these repositories, then add the other as a remote and fetch it, all the trees/blobs seem to get pulled down again, even though they're already in the local repository. It seems like only the commit objects should be fetched, since they're the only difference between the two remotes.

Is this a bug in git? 

-Adam