I've created a new business enquiry report to show the leads and prospects created for the past month. Whilst most appear successfully on the report, some are showing in duplicate or triplicate. (Jon Druitt attached)

I have re-created the report from scratch selecting the report to create based on New Business Enquiry Created Time, but still get these duplicates which distort the data. 

Why am I getting these and how do I stop them?

Alex