There are many ways of detecting duplicate data in your tables. One of the quick and easiest methods I have found is to use the below syntax.
It will return a list of duplicate entries by the field that you’ve chosen and the number of duplications. This then allows you to use that list in a subsequent query to return all of the data within the duplicate row. From there you can then make a decision about what you want to do with the duplicate data.
SELECT EmployeeID, COUNT(*)
GROUP BY EmployeeID
HAVING COUNT(*) > 1;