mysql> create table t1(f1 char(25), unique(f1)) default character set='cp1251';
Query OK, 0 rows affected (0.07 sec)
mysql> insert into t1 values('е');
Query OK, 1 row affected (0.19 sec)
mysql> insert into t1 values('ё');
Query OK, 1 row affected (0.09 sec)
mysql> select * from t1;
+------+
| f1 |
+------+
| е |
| ё |
+------+
2 rows in set (0.01 sec)
mysql> alter table t1 convert to character set utf8;
ERROR 1062 (23000): Duplicate entry 'ё' for key 1