Yeah, this is a good idea, that's why Digg has become a main stream place where just a few well known URLs get submitted and front page all the time while the little guy gets no attention.

We shouldn't penalize such submissions though, that's wrong as well. They should just count less. Like 50% or only the points above 20 should count.

Or something like this:

if url already popular 10 times this week

karma = karma - 10