I have a two year old established site that's only had search engine friendly urls. Just recently i'm noticing that google is indexing my site with "?page_id=384" style urls in addition to the search engine frindly ones. Yesterday there were 8 of these urls in googles index and today there's 48 of these duplicate pages indexed.
I kind of saw this coming about a month ago. I was checking my latest visitor stats and noticed that google crawled nearly every page on my site at once with the '?page_id' url structure.
What can be the cause of this problem? I can't think of any reason why google would even know these urls existed on my site - I didn't even know about them. There's no links pointing to them or people visiting the site through urls like these. And even if you visit the site through one of these ?page_id urls, every link on the site has the correct search engine friendly url.
I'm running wordpress 2.2.1 and the 'Sitemap Generator 2.7.1' plugin - which isn't including any of these crazy ? urls in the sitemap.xml file.