Additional fields should be disabled if already requested in the event

[yankedev]

When editing an event:
Adding an additional field: if the field is already present in the list, it should be disabled from selection

[Praitheesh]

I'm happy to work on this issue.
I suggest rather hide the field from list if already selected , It would be more appropriate to validate unique Field name and Label. Because some time admin may need to add same field again and again then he/she wish to modify the Field name or Label. example for Mobile number / Home number / Fax number.