If the content is of high quality, there can be only 2 possible reasons on high level - #1. duplicate content or multiple thin pages within site and #2. Bad link profile. As you have pointed out, there are other sites republishing your content - but that's not termed as duplication (it's a case of syndication, which is perfectly normal). There are few things you should take care of while doing syndication - like using nofollow backlinks, referring to source etc. I have been helping clients successfully recover their sites for quiet a long time now and have never seen syndication as an issue. So if you want me to take a look, feel free to schedule a call and I'll take a look.