I mentioned this in a previous post (~Dropped 4 pages for 1 search term, other search term remains at top) but thought it warrants its own post.
We have been working (SEO) on a site for around a month now and it has only just emerged that the sites owner also has put up 2 mirror copies of the site we have been asked to work on under different domains.
These other 2 sites are pretty much ignored by Google. is there likely to be any detriment to the site we are working on or is Google ignoring the other 2 sites it's sole way of dealing with this issue.
Google does not like duplicate sites regardless of which domain they are on. I do use testing sites but I make sure the SE's will never find them (new folders on a domain, not linked anywhere and I use a .htaccess rule on the main folder stating never to cache anything inside).
I think you need to be sure that "pretty much ignored" is "completely ignored" or you may run the risk of being penalised.