This tutorial will explain session definition. 

Session definition is found in the Tracking & Controls section when setting up an offer or editing an offer. 

Session definition - How Everflow will classify a user as unique. The most accurate way to do this is through a cookie. We also support IP and IP + User Agent. If the session definition is set to 24 hours that would mean that any user that comes back to that offer within 24 hours will be considered a duplicate user and will be taken to a dead page. 

Duplicate Click Filter - If you enable "Duplicate Click Filter" this will give you the open to send that user somewhere. You can send them to a "Fail Traffic" offer or "Block" them.