ColourOfSpring - 3:03 pm on Aug 5, 2013 (gmt 0)
Many SEO experts recommend having TWO sites, one for desktop, one for mobile. I disagree. You should have ONE site that is responsive to handle both.
Otherwise, you're asking for a dupe penalty.
If you mean two URLs per page (www.site.com, m.site.com) I see your point. But don't forget you can deliver content in a different way for the very same URL (e.g. www.site.com/page.html), depending on useragent.