This post gives a short explanation of NetFlow duplication problem, why it is important and how to overcome it. Network admins think about their data and if it is actually correct and deduplicated. And rightfully so! This should not be on their list of concerns - NetFlow analyzer should include out-of-the-box solution to this problem. Unfortunately, this is often not the case because exporting and processing NetFlow data depends on the network and its complexity.
In general, there are two ways duplication can occur: