MySQL – Make an existing Field Unique

by 
ALTER IGNORE TABLE mytbl ADD UNIQUE (columnName);

For MySQL 5.7.4 or later:

ALTER TABLE mytbl ADD UNIQUE (columnName);

As of MySQL 5.7.4, the IGNORE clause for ALTER TABLE is removed and
its use produces an error.

So, make sure to remove duplicate entries first as IGNORE keyword is no longer supported.

Reference

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