So, you're carrying on about your business and suddenly something substantially similar to you pops up and trashes your life, making you persona non grata in the world you inhabit. You cease to exist; your presence is wiped out.

This is the perfect analogy for duplicate websites. Take the example of wanting to increase your reach and leverage by creating duplicates of your site and hosting them on different domains. Your original site's domain is www.seodoppelganger.com, and you host a copy of that site on www.eviltwinseo.com.

The hope is that by multiplying the instances of your site you will saturate the web space that its aimed at, pushing it higher up the rankings, getting more traffic and increasing conversions. A very, very long time ago this may have been effective, but in the world of search engine algorithms and data interrogation this approach can spell doom for both sites.

Duplicate sites are treated as spam, and spam is penalized. The outcome is that the duplicates cancel each other out, and neither can perform well in the SERPs. This also applies to sites that are hosted across TLD (top level domain e.g. .com .tv .net etc.) ranges, so www.seodoppelganger.com , www.seodoppelganger.tv and www.seodoppelganger.net would get penalized too.

If, for whatever reason, you have duplicate sites it would be wise to implement 301 redirections for each and every page, so www.eviltwinseo.com/fixed would go to www.seodoppelganger.com/fixed. This should be carried out with TLD based duplications too, so www.seodoppelganger.tv/fixed and www.seodoppelganger.net/fixed would go to www.seodoppelganger.com/fixed. With the redirections in place you send a clear message to the search engines that you're not trying to hoodwink them.

Duplicate content, such as a couple of paragraphs (or pages) spread across many sites will also invoke a penalty. This poses a problem for affiliates, resellers, or anyone in a channel using your collateral. To avoid this you should ensure that, for example, syndicated content contains links to the original blog. This acts as a flag to the search engines that everything is above-board.