Google's Matt Cutts released a Webmaster video discussing the impact of duplicate content, if the same website is on IPv4 and IPv6.
The question asked was: 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?
Explaining the difference, Matt Cutts said that IPv4 is an IP address with four octets and IPv6 has six identifiers. So, you are serving up same content on IPv4 and IPv6, which won't be tagged as duplicate.
Cutts also said that it is a question similar to having content on dot PL and on dot com. He said that spammers rarely buy multiple domains on different country level domains to have that type of experience. Normally, if your site is on multiple country domains, Google don't consider the duplicate content. So, having same content on both IPv4 and IPv6 is absolutely fine.Matt Cutts on Having a Website Available on Both IPv4 and IPv5 & Dupicate Content Issue!,