What do you mean? The search engine spiders just go through every link they can find and index as much as they can. If they crawl a page that's been indexed, then the most recent copy replaces the older one, that's all.
as Dan said, google dsnt check whether a page was indexed earlier or not, while crawling your site. if it is coming to your site and crawls such page, it just replaces the previous one. it's that simple
Its an interesting question since none of know how exactly search engines work internally. We are sure about few things though that first the crawl it, get some idea and than index it. That's why you can see many times your new site has been crawled(awstat shows) but still you don't see any page indexed (cache: page_url). Google do maintain Primary and secondary indexes, so they like to crawl/index everything but they put important stuff in primary index. Do note that they also de-index lot of pages. I am also pretty sure internally they do maintain older copies of pages.
sounds interesting esp to a newbie like me..it seems have to know more of this stuff well anyway thanks for this new knowledge.. Profitimo