I'm working on two applications, a client and a server. Currently I have both of them checking to make sure user submitted value is legal. They check against duplicate user names and fields with nothing in it. Because I have it on both the client and the server I'm given a bit of redundancy, but I'm wondering what price I'm paying for it. I obviously want to have data validation on the server, or someone could modify the client to submit invalid data. Obviously if I only had it check once it would be faster. Is it possible to validate your data too much?