Webinar registrations - one registration repeated 30+ times

[KarenWild]

We ran a webinar recently and I uploaded the registration/attendee list to Zoom which worked fine. 

When we ran the webinar however, about 30 people came up with the same attendee name.  The attendee report generated afterwards also did the same.  It listed the same person 30+ times with different log in/out times but we can't differentiate who was who (it was not the same person).  There are 30+ people who were registered but showing with no log in/out times.  

That means that there are 30+ people who I can't confirm if they actually completed the webinar or not hence they don't get a certificate for completing the training.  
Has anyone come across occurring before?  I don't think it was anything that I did as the registration list was uploaded correctly.  I don't want this happening again as some participants are going to be upset that they aren't getting a certificate.