Is it benefit to create a index key if unique key already exists on the same columns?
More discussion on building indexes:  http://mysql.rjweb.org/doc.php/index_cookbook_mysql

Furthermore your table smells like a many:many mapping table??  Here are several tips on efficiency for such:  http://mysql.rjweb.org/doc.php/index_cookbook_mysql#many_to_many_mapping_table