Hopefully, this short advice could help everyone to fixed their problem with duplicate content.

if the site is under a current Google penalty, which you seem to imply below, then you need to correct all the issues with the site THEN submit a reconsideration request. Only then will you trigger a manual review of your site by a Google employee and possibly get the penalty lifted.

Same thing goes with your site suffering from duplicate content. You need to REMOVE ALL DUPLICATE CONTENT, add UNIQUE CONTENT to the site AND then submit a reconsideration request telling Google to review the site. Also, it may take several months and multiple submitted reconsideration requests for you to solve all the problems.