A Google Webmaster Help thread has one webmaster who is upset that Google lists the site under one domain and has the cache listed under a different domain. What this webmaster finds out is that Google not only has different domains with the same content but tons of different URLs with the same content.

Canonical issues across different domains or within the same domain name can cause many issues for Google and thus your rankings.

Pierre Far from Google shared some other examples, outside of the picture above. He then explained why this is an issue:

Pierre then sums it up with this line:

Canonical issues can be handled by Google, but good webmasters make sure it is handled for Google.

Forum discussion at Google Webmaster Help.