A thread sprang up last month on the Google Webmaster Help forums with an interesting issue about duplicate content from sites harming rankings in instances where the duplication may have, in fact, been appropriate.
The particular problem was for two sites that required separate, but incredibly similar, content for an Irish audience and a UK audience. Each one was hosted on a country specific domain, but due to the overlap between the UK and Ireland, the Irish site was appearing in the UK search rankings.
Because of the duplicate content showing up in one set of rankings, both sites dropped in rankings.
What then follows is some helpful advice from two contributors on how to avoid this problem, the best option being to utilise sub-domains so that the duplicate content is contained within one place.
The highlight of the thread is when ‘JohnMu’, a Google employee, enters to outline the company’s policy in these matters, and the difficulties they face when deciding what to do. He mentions that normally in these circumstances, one site will be removed from the rankings so that the users are not faced with duplicate entries.
If you care to check the two websites listed by ‘Mr Code Red’ and attempt to find them in Google, it appears that the measures ‘JohnMu’ talked about have been taken. The Irish site does not appear, but the UK one does.
From this, we can see that overlap between search engines has to be considered during SEO work. We’re also reminded of the fact that research before hand can stop these problems from even cropping up in the first place.
A quick read and potentially useful. So, as JohnMu says, “Hope it helps!”