Primary index issue

[JAIME Martin]

Now, I´ve found somthing that looks like a very big bug in MySQL 5.5.11, so before to think that choose MySQL is a bad decission, I prefer to ask the people that has a better knowled about MySQL...

mysql> show index from historico;
+-----------+------------+-----------------------------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+-----------------------------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| historico | 0 | idx_Historico_Clave | 1 | IdTerminal | A | 810 | NULL | NULL | | BTREE | | |
| historico | 0 | idx_Historico_Clave | 2 | Clave | A | 394315658 | NULL | NULL | YES | BTREE | | |
| historico | 1 | idx_historico_FechaRecepcion | 1 | IdTerminal | A | 810 | NULL | NULL | | BTREE | | |
| historico | 1 | idx_historico_FechaRecepcion | 2 | FechaRecepcion | A | 394315658 | NULL | NULL | YES | BTREE | | |
| historico | 1 | idx_historico_idterminal_fechagps | 1 | IdTerminal | A | 810 | NULL | NULL | | BTREE | | |
| historico | 1 | idx_historico_idterminal_fechagps | 2 | FechaGPS | A | 394315658 | NULL | NULL | YES | BTREE | | |
+-----------+------------+-----------------------------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
6 rows in set (50.31 sec)

As it is possible to see, the "historico" table has a unique index named "idx_historico_clave" over the "idterminal" and "clave" fields.
The question are how is it possible the following:

mysql> select idterminal, clave from historico where idterminal = 38720 and fechagps = '20170611230057';
+------------+------------------------+
| idterminal | clave |
+------------+------------------------+
| 38720 | G20170611230057P102355 |
| 38720 | G20170611230057P102355 |
+------------+------------------------+


Aditional information:
- Table historico is a huge table, it has over 300.000.000 registers
- Table historico is partitioned by HASH(idterminal) in 64 partitions
- The database who has the problem is a "copy" of a production data base, obtained throw a "total" mysqldump (mysqldump -uroot -pXXXXXXX --skip-lock-tables datos historico, and then inject into the database using "mysql" command line.
then, I´ve done a partial backup of the production database using "mysqldump -uroot -pXXXXXXXX --skip-lock-tables --insert-ignore datos historico -t -w"fecharecepcion>'20170619230000'", and import in the copy database
Then, I´ve done a partial backup of the production database using "mysqldump -uroot -pXXXXXXXX --skip-lock-tables --insert-ignore datos historico -t -w"fecharecepcion>'20170619230000'", and import in the copy database

So, when I try to do "alter table historico rebuild partition p0;" I get the following error:
ERROR 1062 (23000) at line 1: Duplicate entry '38720-G20170611230057P102355' for key 'idx_Historico_Clave'

or if I try to do something like 'insert into historico (idterminal, clave) Values (1, 'pp'); twice, the second time I get a similar error message