MySQL allows its users to create several indexes that might be the same (duplicate indexes) or partially similar (redundant indexes) in its structure. Although duplicate indexes have no advantages, there are some cases where redundant indexes might be helpful. However, both have disadvantages. Duplicate and redundant indexes slow down update and insert operations. As a result it is usually a good idea to find and remove them.
Doing this manually would be a time consuming task, especially for big databases and that is why there is a utility to automate this type of task: mysqlindexcheck.