The whole area of duplicate content causes a lot of worry - and many times, it's unnecessary worry. At SES Chicago, Google's Susan Moskwa spoke and put duplicate content issues back in balance. A lot of what she offered is not news to experienced SEOs, but she did get into some relatively new areas - especially addressing translated versions of content on international sites (and that is NEVER considered duplicate content).
Susan offered this summary:
When should you not care about duplicate content?
Common, minimal duplication.
When you think the benefit outweighs potential ranking concerns. Consider your cost of fixing the duplicate content situation vs. the benefit you would receive.
Remember: duplication is common and search engines can handle it.
There was one other point that is important to get - master this idea!
The myth of the duplicate content penalty. A lot of people think that if they have duplicate content that they'll be penalized. In most cases, Google does not penalize sites for accidental duplication. Many, many, many sites have duplicate content.
Google may penalize sites for deliberate or manipulative duplication. For example: auto generated content, link networks or similar tactics designed to be manipulative.
Thanks for the timely posting of this reference. I'm actually days away from rolling out an exact duplicate, in another language, of a site for a bilingual client who serves her clients in 2 languages. Regardless of how Google handled it, it was going forward, but this article provides peace of mind knowing that search engines are treating it as indicated in the article.
Each page of the the 2 versions cross-reference each other in page header tag as follows: <link href="https://www.example.com/" hreflang="en" rel="alternate" title="English Translation Version" type="text/html" />
I had actually found out about using that tag somewhere else on WW, don't remember who or where but thanks to whoever posted it. The header rel="alternate" is for the search engines and additionally each page also has a visible cross-referenced link for visitors near the main navigation menu.