We are running an information website since 2003 using php. All sites were indexed well, but in March 2006 google bot began to index our pages with PHPSESSID, which we need use for our users as leaflet. In July we figured out that our pages in the index are growing extremly and we saw that google bot began to index the dupe pages. We directly fixed the problem as said at google webmaster resouce set in robots.txt
we also fixed our scripts so that the google bot doesn't get a PHPSESSID when he spiders the sites, because its no extra content, only for better usage for our users.
Our website has nearly 55.000 unique pages, but site: DOMAIN PHPSESSID said there were 230.000 pages, so there were a lot of dupes. At the first time it works well, within 3 weeks these PHPSESSID went down to 15.000 pages and everything was looking fine. Then end of August a data center square came and within one day the PHPSESSID pages grow to 82.000 pages, since this day the number of 82.000 never changed. Google doesn't delete any page of this NOT WANTED pages.
Actually we wouldn't care about those results, but two weeks after the "data backfall" our search engine rankings and traffic lost 80%. It looks like because of this dupe pages or of the rapid growing of new pages, google kicked our rankings down. But the main technical problem are these ghost pages, we can't do anything.
Have you ever heard about this special problem? What can we do now but waiting months or years?