If you require adding a UNIQUE key to a MySQL table, but the desired column contains duplicates, this stored procedure will assist in raising cardinality by appending random numbers to the end of a duplicate record.

deduplicate-column.sql

DELIMITER $$

CREATE PROCEDURE `sp_utility_deduplicate_column` (IN table_in VARCHAR(64), IN unique_in VARCHAR(64), IN nonunique_in VARCHAR(64), length_in INT(11))
	BEGIN
	
	/*	sp_utility_deduplicate_column

		IN	table_in	The name of the table to execute deduplication on
		IN	unique_in	The PRIMARY KEY for table_in (auto-incremented integer recommended)
		IN	nonunique_in	The column that contains duplicated values (INDEX recommended)
		IN	length_in	Length (number of digits) to add to the end of the column value
		
		WARNING: If the value of a column that has a duplicated record is near or at the maximum length 
			 its data type, the initial attempt to add significant randomness may fail, triggering a
			 second pass. This second pass will remove a number of characters from the end of the
			 value equal to length_in, which could alter the core value!
		
	*/
		
		DECLARE duplicates INT(11);
		DECLARE iteration TINYINT;
		SET iteration = 1;
		
		deduplicate: WHILE 1 DO
			SELECT COUNT(*) INTO duplicates FROM table AS a JOIN table AS b ON a.column = b.column WHERE a.id <> b.id;
			
			IF duplicates > 0 THEN
				SET @query = CONCAT("
					UPDATE ",table_in," AS a
					JOIN ",table_in," AS b ON a.",column_in," = b.",column_in,"
					SET a.",column_in," = 
						CONCAT(
							IF(iteration > 1,
								SUBSTRING(a.",column_in,", 1, CHAR_LENGTH(a.",column_in,") - (",length_in," + 1)),
								a.",column_in,"
							),
							' ',FLOOR(1 + (RAND() * 50))
						)
					WHERE a.",unique_in," <> b.",unique_in
				");
				PREPARE STMT FROM @query;
				EXECUTE STMT;
				DEALLOCATE PREPARE STMT;
				
				SET iteration = iteration + 1;
			ELSE
				LEAVE deduplicate;
			END IF;
		END WHILE;
	
	END$$

DELIMITER ;