Fix duplicate indexes

Some indexes are left-prefix of others indexes.

"If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to find rows. For example, if you have a three-column index on (col1, col2, col3), you have indexed search capabilities on (col1), (col1, col2), and (col1, col2, col3)."

Source : http://dev.mysql.com/doc/refman/5.5/en/multiple-column-indexes.html