How to check that field value is unique

I have a base with thousands of records. I have a simple formula to populate a new field in those records by concatenating some already existing data.

I cannot control or change what those source data are.

I need to check if the new values in the new field are unique, and tag any records where that value is not unique so I can apply a secondary process to manage duplicates if and when they arise.

I need to apply this process to my thousands of existing records, plus also apply it to every new record written to the table, so I can tag any duplicates and then run my secondary process to manage those duplicates.

What’s the best way to achieve the uniqueness check and tag any duplicates?