Peter I disagree with that:
"Duplicate content doesn't cause your site to be placed in the supplemental index. Duplication may indirectly influence this however, if links to your pages are split among the various versions, causing lower per-page PageRank. "
Massive duplicating to put your site down like this scum did at www.natural-elements-online-si.com
or do a search at natural-elements-online-si.com on Google and see how this scrapers use the customer accounts to place their mirror pages. They stolen contents from me, use my keywords, and stolen contents on their meta tag descriptions, destroyed my serp for a while.
My attorney advised me to get as much as I can a lot of proofs of their scams, once I will be finished, I will put these people in court.
If you have any suggestions other than what I am doing, please do.