If you need an actual field, then yes you will need to have some daily process like a button to click to merge in the unique values to a summary table. I have not tested, but you may be able to make a saved table to table copy where the values of the field are "merged' into a table where the Key field is the field in question, and have it thus add new records if needed. Then a user would push that button from time to time to update the master table of unique fields.
The button would used the urlroot() & "db/" &[_DBID_MY_TABLE_ID] & "?act=API_RunImport&ID=10" as part of its formula ..
