Tell PG: Preventing Double Posts

You might have noticed that sometimes, the same story is submitted twice. Since I haven't dug through the source code of HN, I'm not sure why, but I noticed that the system looks at www.example.com and example.com as different URLs. This means that if I submit a story with a www. in the link, and someone submits the same story from the same source, but without the www., it gets submitted. This can be easily fixed in other languages, I don't know about Arc (it shouldn't be a problem, though).