Had a 10 year old site that was hit with Panda on the first international rollout. All "original" content - i.e. stuff that had been written specifically for the site. Most of the content ranked pretty well and was scraped to hell.
The key part is in bold there. "Original" doesn't mean "good", and it's that quality angle that Panda is all about.
I left the site for the better part of a year to see if anything would change (it didn't). Early January I worked on a fix (removed about 10 articles which I deemed to be of a low quality and didn't really deserve the rankings they previously had). Site recovered fully during the next refresh and has been fine since.
Duplicate content is a standard go-to solution for SEOs for a lot of problems, and certainly may be a factor in Panda.
But what could also be quite likely with the OP's situation is that there is absolutely nothing "wrong" with the site in terms of bad SEO stuff (dup content, canonicalisation, keyword stuffing, etc etc). I.e. nothing that "triggered" a penalty.
It could just be that Google has shifted the goalposts in terms of what they deem to be of a good quality and regardless of "authority" or "white hat SEO" of the OP's site, it is still lacking on that front.
The real big disruption that Panda caused is that even if you've been purely "white hat" over the years, junk content (even well-written junk content) that's purely and blatantly for SEO purposes can ruin you.
Time to re-assess what constitutes "quality content" IMO, and for a Panda solution, you need to be brutal about what content you keep and what you purge.
To put it into context, I'd label some (the minority) of guest posts on SEL as being of a low quality. They may be well written, sometimes by seasoned professionals, but they can be thinly veiled SEO / marketing efforts at times.
On the other hand, Barry over SER does pretty succinct short posts - sometimes less than 50 words.
"Traditional" SEO would probably say that the more lengthy articles are better - it's something we've all believed and advocated. But in a Panda world, quality outranks (or at least, can "override") length, source, authority and so on.
PS - I'm not saying dup content isn't an issue BTW; just that it might be less of a cause of problems than people generally imagine.