We understand that this is an important feature. While we currently don’t have it, you can use automation tools like Zapier to get a temporary solution. Here is an example of a simple workflow that should do what you need: it runs when a row is created or updated and checks if a record already exists. If it does, it updates a row by adding “Yes” to the ‘IsDuplicated?’ field.
There is also a suggestion from @frederikdc: Remove duplicates - #2 by frederikdc.