Is an explicit KEY redundant if it is covered by a UNIQUE KEY?
Yes, you can drop it because baz and baz_3 have identical leading columns. This scenario is known as having redundant indexes.

However, be careful that the key baz is not being used to support a foreign key constraint if the table is InnoDB. See my post Do I have duplicate key indexes?

If the table is MyISAM, go ahead and drop it with confidence. The MySQL Query Optimizer will use the UNIQUE KEY baz_3 for query optimization in its execution plans.

If you want to locate all tables with redundant indexes, see this post : MySql - find redundant indexes