I think you misunderstood my point.

My point is that corporate users behind a load balancer doing round robbin will have the separate posts come from sequential IPs, therefore your lookup by IP will fail to see the previous post for that user. While this is not common today, it is becoming more and more common as time goes by.

Given the speed of double-posts and the frequency of posting overall, I also believe that it would work fine for anonymous users. If it would not then you could beef up the logic to take care of them specially.