how can I guarantee that a repeat visitor won't cause the server to repeat an action?

Question

I want the server to make a change to a database when a first time user visits. I don't want it to be easy for a user to pretend to be a first time visitor again (such as deleting his cookie). Does anyone have ideas on how to accomplish this?

Answer 1

Track a user's HTTP header data. Note that this will not be accurate if users at the same IP are using computers with identical configurations.

Answer 2

Short of making them login there is not much. If you log ip address you will not be able to distinguish behind NAT.