Google released a new Webmaster Help video today discussing IPv4 and IPv6 with regards to possible duplicate content issues. To make a long story short, there are none.
Google's Matt Cutts responded to the following user-submitted question:
As we are now closer than ever to switching to IPv6, could you please share some info on how Google will evaluate websites. One website being in IPv4, exactly the same one in IPv6 - isn't it considered duplicate content?
"No, it won't be considered duplicate content, so IPv4 is an IP address that's specified with four octets," says Cutts. "IPv6 is specified with six identifiers like that, and you're basically just serving up the same content on IPv4 and IPv6. Don't worry about being tagged with duplicate content."
"It's the similar sort of question to having content something something dot PL or something something dot com," he continues. "You know, spammers are very rarely the sorts of people who actually buy multiple domains on different country level domains, and try to have that sort of experience. Normally when you have a site on multiple country domains, we don't consider that duplicate content. That's never an issue - very rarely an issue for our rankings, and having the same thing on IPv4 and IPv6 should be totally fine as well."
More on IPv6 here.