Redundant Interface: An interface that duplicates functionality provided by some other interface.

e.g.
Doorknobs that can be turned clockwise or counterclockwise to open the associated door.
In Windows, being able to close an application by clicking on the X, selecting file|close, pressing alt-f4 or selecting End Task from the Close Program dialog.

Frequently, redundant interfaces are taken for granted. Despite this, many people will violently oppose creation of a redundant interface. It is my belief that redundant interfaces allow people to choose the interface that seems most natural to them.