TOPIC: Duplicate Registration Message

The anti duplication feature when set works but returns a blank page.
It needs say something like
"[UserName], you previously registered on 'Register Date' for the [EVENT_NAME] being run on [EVENT_DATE].
This should be editable via DT Register/Configuration / Messages
Thank you

First, the duplication prevention feature is not connected to a user, but only to an email address. So after submitting the registration form, it checks the email address, then will notify in a popup alert that you are already registered. That text comes from the language file:

DT_ALREADY_REGISTERED=You have already registered for this event

However, I think you are actually talking about something else. A "blank page" means you are supposed to be getting some message that you have not yet configured in the messages tab. This could be the private events message, overlapping message, prerequisite message... You can get a clue by looking at the url in your browser. &task=.....