Why pt-duplicate-key-checker suggests removing a composite index? Here's a snippet from the output of the Percona Tool pt-duplicate-key-checker which searches for redundant indexes:

# Key myidx ends with a prefix of the clustered index
# Key definitions:
#   KEY `myidx` (`bar`,`foo`)
#   PRIMARY KEY (`foo`),
# Column types:
#         `bar` mediumint(8) unsigned not null default '0'
#         `foo` mediumint(8) unsigned not null auto_increment
# To shorten this duplicate clustered index, execute:
ALTER TABLE `mydb`.`mytable` DROP INDEX `myidx`, ADD INDEX `myidx` (`bar`);


Why does the tool suggest this? Can't the original composite index be useful? 

As far as I understand, an index on bar would be worth of deletion given a PK (bar,foo), but it is not the case here.
The primary key is a part of any secondary key in InnoDB.