How to stop multiple sign ups for events

We have found that people often sign up for our online events through ONX when we first market it and then often forget and sign up again at a later date (we have even had people making multiple donations). Does anyone know if there is a way to stop people signing up for a event more than once, or whether we just have to make sure that when processing we are checking the current guest list first and then ensuring that we only add people once (which is tricky when you have hundreds of people signing up). Thanks.