The way I've heard it argued is that as more and more things happen in the client, you want some level of data validation there. But as we all know, it's not en

The way I've heard it argued is that as more and more things happen in the client, you want some level of data validation there. But as we all know, it's not enough to just validate the data in the client - you need to do it on the server too. So hence code duplication.