somewhat open ended question here as I am mostly looking for opinions.  I am grabbing some data from craigslist for apt ads in my area since I am looking to move.  My goal is to be able to compare items to see when something is a duplicate so that I don't spend all day looking at the same 3 ads.  The problem is that they change things around a little to get past CL's filters.
I already have some regex to look for address and phone numbers to compare, but that isn't the most reliable.  Is anyone familiar with an easy-ish method to compare the whole document and maybe show something simple like "80% similar"?  I can't think of anything offhand, so I suspect I'll have to start from scratch on my own solution, but figured it would be worth asking the collective genius of stackoverflow :)
Preferred languages/methods would be python/php/perl, but if it's a great solution I'm pretty open.
Update: one thing worth noting is that since I will be storing the scraped data of the rss feed for apts in my area (los angeles) in a local DB, the preferred method would include a way to compare it to everything I currently know.  This could be a bit of a showstopper since that could become a very long process as the post counts grow.
You could calculate the Levenshtein difference between both strings - after some sane normalizing like minimizing duplicate whitespace and what not. After you run through enough "duplicates" you should get an idea of what your threshold is - then you can run Levenshtein on all new incoming data and if its less-than-equal to your threshold than you can consider it a duplicate.
What is the easiest method to compare large amounts of similar text?