An event has been added to an Event Registration Form part, but displays two times. When the user attempts to delete one of the copies, a Runtime Error message appears.
This issue has been observed as a result of two users editing the same event form at the same time. As a result, when one user removes or adds an event while another user does the same, it causes an internal error. Once this occurs the Event Registration Form will see that the event is listed twice when it should only be listed once.
If this occurs, delete the form and create a new one from scratch as the event will become locked in the original form.