Is this a formal definition? Does duplication harm my site noticeability? Should I do something about it?

The same material published two or more times on the one website is at least duplication. A single small paragraph, statement or diagram is not considered a problem but entire articles, lists, extracts of tables are. Duplication hinders categorising websites for the sake of various indexation , ranking etc.

At the moment duplication is not doing any harm to your site. The strategies are being developed currently as a collaborative effort by a number of website related organisations and companies. Google is undoubtedly well up there and they have released an informative video with Chris Coutts discussing the issue. Well worth the watch if you have an interest. Chris has reassured web-authors that there most definitely is no panic to remedy duplication and that information will be released in a timely manner as the processes are finalised and methods for deployment are agreed upon. There are current methods available with surprisingly simple lines of code. You can view this on Google's resource pages somewhere??

As mentioned there is no panic or urgency to act yet. It's probably worthwhile just getting your head around it. Examining your own sites and identify signifiacnt duplication of content. I'm sure various Q&A's will emerge if tey haven't already and specialist panels and forums will be seen to advise.