I understand this question is asked frequently but my case is slightly different.
I work for a website allowing users to have their own blogs and it might happen that one of the authors might decide to leave my website to self-host his blog.
I want to take a friendly approach when this happens, that is, I will help the blogger in the export/import of data.
But what about duplicate content issues? If the blogger published 100 articles on my website, 100 links will be duplicated on two domains, mine and his:
http://www.mydomain.com/blogger/blog-name/article
and
http://www.bloggersdomain.com/article
How shall I tackle this problem? I mean both URLs will have the same, exact content and that is because I will not remove the articles from my website.
Will I be penalized by Google because of duplicate content?
bloggersdomain
is likely to have the harder task of getting indexed for this reason, unless you are willing to set up 301 redirects to their new site?