Have a "Find Duplicate" feature like Classic did.
Matt Cox commented
We have an app script that runs and compares the key info that we use to create our record IDs. If it finds it is a duplicate i just puts the word "duplicate" in a text field. Then i have a view that only populates if a record has the word duplicate in that field. With the View shares or zapier you can set it up to notify you when there is a duplicate if it's not something you want to have to remember to monitor.
While there is not a specific tool that will identify duplicate records, you can create a summary view that will highlight any duplicates you have.
The summary view would Group and Count the Record ID of the table. You can then sort the Count column descending to show any counts over 1 at the top of the view. Any count over 2 shows the duplicates.