This is one that I still keep seeing around day after day online. People seem to think that having duplicate content on your site will see it plummet out of the rankings. For example the CMS this site is built on, e107, has duplicate content all over the place. For example news pages can appear under several URLs. This does not cause Google to some how penalise the site!
Google make no secret of this:
Webmasters often wonder if the duplicates on their site can have a negative effect on their site’s traffic. As mentioned in our guidelines, unless this duplication is intended to manipulate Google and/or users, the duplication is not a violation of our Webmaster Guidelines. The second part of my presentation illustrates in greater detail how to deal with duplicate content using canonicalization.
Sure, there is a down side. Sometimes I find that that Google does not index the right e107 page – no problem, just setup your robots.txt file correctly. See here for a e107 robots.txt tip. And you might find people link to different version of your page – hopefully e107 0.8 will sort out some good permalinks to clear this up.
But Google will not penalise your site, they will pick one page and they will rank it!