When you do a search on google and come to the end of the results there will be a link and text like this:
"In order to show you the most relevant results, we have omitted some entries very similar to the 35 already displayed.
If you like, you can repeat the search with the omitted results included." If you click onthe link, the &filter=0 gets added to the search url and shows all of the sites that Google didnt show on the first pass - at one time Googleguy confirmed here (I believe around two years ago) that this was an example of the dupe content filter - although I would think that it would also be used for other types of filtering?
Primarily it would knock out the sites that have more than two entries for that keyphrase as Google will only show 2 entries from the same domain in its first pass results.