Finding duplicate values in MySQL

Do a SELECT with a GROUP BY clause. Let's say name is the column you want to find duplicates in:

SELECT name, COUNT(*) c FROM table GROUP BY name HAVING c > 1;

This will return a result with the name value in the first column, and a count of how many times that value appears in the second.

Building off of levik's answer to get the IDs of the duplicate rows you can do a GROUP_CONCAT if your server supports it (this will return a comma separated list of ids).

SELECT GROUP_CONCAT(id), name, COUNT(*) c FROM documents GROUP BY name HAVING c > 1;