Rather than trying to avoid duplication, I'd suggest first evaluating your goals for the site in general (sales? awareness? education?). Your mobile site should work toward those goals as well, since some percentage of people may only ever see your site on mobile. Next, inventory the content you have and determine whether it lends itself to consumption on mobile. A desktop site that contains lots of visual schematics or lengthy whitepapers may not translate well to mobile. However, if your desktop site features short videos, audio files, case studies, and brief articles, by all means place those on your mobile site. Multimedia content presented in an easy-to-navigate layout does great on mobile, particularly with smartphone users. Just think of all those people glued to their phones while they are in transit, waiting in line for anything, or at dinner with disliked in-laws.
Prioritize the goals for your site, make sure you are meeting them on mobile, and don't worry too much about duplication.