In order to make a field unique in your existing table you can either run a sql statement or use a visual sql client like HeideSQL.
This is an example on how to make your emp_id field unique in your existing employee table.
ALTER TABLE `employee` ADD UNIQUE INDEX `emp_id` (`emp_id`);

This is the return error for duplicate entry
SQL Error (1062): Duplicate entry ‘ID007′ for key ‘emp_id’
Nice info..Thanks