Error 1062: Duplicate entry for key

CREATE TABLE t1 (
  id INT AUTO_INCREMENT PRIMARY KEY,
  f VARCHAR(10) UNIQUE
);

INSERT INTO t1 (f) VALUES ('a'),('b');

SELECT * FROM t1;
+----+------+
| id | f    |
+----+------+
|  1 | a    |
|  2 | b    |
+----+------+

INSERT INTO t1 (f) VALUES ('b'),('c');
ERROR 1062 (23000): Duplicate entry 'b' for key 'f'

ALTER TABLE t1 DROP INDEX f, ADD INDEX (f);

INSERT INTO t1 (f) VALUES ('b'),('c');

SELECT * FROM t1;
+----+------+
| id | f    |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | b    |
|  4 | c    |
+----+------+