FALSE MYSQL DUPLICATE ENTRY ERROR

By placing an index on the table I get this error

Executing:
ALTER TABLE 'esd'.'sdf' 
ADD UNIQUE INDEX 'index2' ('id' ASC, 'periodo' ASC);

  

Operation failed: There was an error while applying the SQL script to   the database ERROR 1062: Duplicate entry '45526-201810' for key   'index2' SQL Statement: ALTER TABLE esd . sdf ADD UNIQUE INDEX    index2 ( id ASC, periodo ASC)