No. Duplicate content is not a problem, because pages with
?view=mobile are never indexed. These URLs are used only to tell the browser that you explicitly want to see the mobile site. When you install OMF you have 2 templates—the one of your desktop site and the one of your mobile site. However, these are not separate pages. Templates are something that is used behind the scenes, on the server, to generate a page. In theory, you can generate an endless amount of pages for the same URL e.g. if the visitor is using a mobile device, generate this page, otherwise, generate that page. This is roughly what's going on behind the scenes.
Google sees a page on your website as 1 mobile-optimized page (more like 2 versions of the same page than 2 separate pages), which is very good, because it actually allows you to rank higher on mobile searches. The mobile page is only indexed for mobile search and the desktop page is indexed for desktop search.
"But the content on the mobile and the desktop pages is sometimes different"
The fact that the content is not entirely the same on the mobile and the desktop site is due to the third-party extensions and modules that are installed on your website. These are also dynamically used when a page is being generated. Since OMF is built to work with the default OpenCart setup, it only knows about the OpenCart settings and modules. It doesn't know about third-party modules and that's why it can't include them when generating a mobile page. However, you can explicitly enable some desktop modules on the mobile site. Just tick the ones you want to enable in the "Modules" tab of the OpenCart Mobile Framework Administration.