Hello!
In order to dynamically fill the dropdown list with a list of dates without
duplication, I would suggest setting “Type of Content” to “Date”, and then set
“Data Source” to “search for record with date:unique elements”.
This should then give you a list without duplicates.
You may also want to check out the Unique element in a repeating group forum thread, as it contains some similar questions and helpful advice from fellow users!