According to an article on CNET, file sharing company Altnet may begin suing competitors for "violating" it's recently-licensed patent for finding identical files based on a hash of their contents.

This may eventually win the prize for stupidest patent when such a prize is given, considering the whole purpose of hashes is to use them as a representation of the contents being hashed.

According to the article, the previous patent licensees had already litigated it and lost (a case between Digital Island and Akamai ending in December of 2001).

After further investigation, the patent is titled "Data processing system using substantially unique identifiers to identify data items, whereby identical data items have the same identifiers" and the abstract reads:

In a data processing system, a mechanism identifies data items by substantially unique identifiers which depend on all of the data in the data items and only on the data in the data items. The system also determines whether a particular data item is present in the database by examining the identifiers of the plurality of data items

I'm wondering if any of the database manufacturers may be interested in looking at this patent.