Using DNS to Redirect Several Domains into One Single Content. Disaster?

Answer 1

If you are managing the websites via Google Webmaster Tools, it is possible to specify the "primary domain".

However, the world of search engines doesn't stop with Google, so your best bet is to send a 301 redirect to your primary domain. For example.

www.foo.net should 301 redirect to www.foo.com

www.foo.net/bar should 301 redirect to www.foo.com/bar

and so on.

This will ensure that www.foo.com gets the entire score, rather than (potentially) a third of the score that you might get for link-backs (internal and external).

Answer 2

Look into canonical links, as documented by Google.

If your site has identical or vastly similar content that's accessible through multiple URLs, this format provides you with more control over the URL returned in search results. It also helps to make sure that properties such as link popularity are consolidated to your preferred version.

They explicitly state it will work cross-domain.