Hi,
As far as I understand it's not really a question of duplicate content in the SEO meaning. Although all the definitions starting with M are on the M-index page this page is quite different to the pages that contain the individual definitions of the terms that start with M.
A problem on many sites is that the pages that only contain the explanation of one term are very light in terms of content, and that the page with is listing all these terms is generally not very interesting from a user (and search perspective). I don't know your site, so difficult to assess if this is the case
You could make the index page noindex/follow - and just list the terms, linking to the explanation pages. For the explanation pages which are probably the most interesting for users & search engines: try to enrich them by adding more content, like links to articles on your site that use the term, or have more information on the term
Hope this helps,
Dirk