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If the site content is exactly the same Google will see them as duplicate content. If the site content is different Google will see them as separate sites, however you will now have two sites competing for ranking with one another. Google usually sees the m.domain as for mobile but not always. I tend to use a user agent redirect which detects if users are on mobile based browsers and redirects them to the m.domain. So you can keep your normal listing in Google, yet when users click on it on a mobile device they will be redirected to the mobile page.

That sounds perfect - if the user-generated URLs are getting enough traffic, make them permanent pages and 301-redirect or canonical. If not, weed them out of the index.