Like many others here, I read a lot. Consequently, I find that I have already seen many of the links on this site.

With this in mind, you should consider providing a link next to each article that a user could click if he/she has already read this article elsewhere.

This info could then be used to give an idea of how old the article is, and would also perhaps help prevent overlap with reddit/digg, thereby improving the value of your site.