As long as you have permissions as a "User" in Mass Mailing, the option should appear when you open up a mass mailing campaign (can be in any stage, even sent). Click Action>Duplicate. This option is enabled by default and shouldn't require anything additional. This is on Enterprise for Version 10 and 11.

Hi Gregory,

Unfortunately, "duplicate" don t appear on my session, see : http://zupimages.net/viewer.php?id=18/28/xead.png