mysql> create table `tm` ( `name` varchar(255), primary key (`name`)) engine=InnoDB default charset=utf8;
Query OK, 0 rows affected (1.18 sec)

mysql> insert into tm values ('București');
Query OK, 1 row affected (0.01 sec)

mysql> insert into tm values ('Bucuresti');
ERROR 1062 (23000): Duplicate entry 'Bucuresti' for key 1

mysql> delete from tm;
Query OK, 1 row affected (0.00 sec)

mysql> alter table tm modify `name` varchar(255) CHARACTER SET utf8 COLLATE utf8_unicode_ci;
Query OK, 0 rows affected (0.46 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> insert into tm values ('București');
Query OK, 1 row affected (0.01 sec)

mysql> insert into tm values ('Bucuresti');
ERROR 1062 (23000): Duplicate entry 'Bucuresti' for key 1