Control duplication is common in airplanes. This is usually called dual controls and does not imply that there are two of each control in the airplane. Some of the controls are placed in a central area where they can be accessed by either pilot. Other controls such as instruments are placed so that either pilot may see the instrument, but one or the other pilot may have a more direct view of it. Other times, there may be only one control for the function. The decision on how much of the airplane’s control system is duplicated is up to the designer in compliance with the aviation regulations.
Why should there be any duplication at all? Duplication is not required in over-the-road buses where the driver will be at the controls for similar time periods as a pilot. Locomotives also have a single set of controls. Not even all aircraft have dual controls.
There are several reasons for having dual controls in aircraft. An obvious reason is for instruction. The first flights for a student pilot are times of demonstration and then execution. The instructor needs controls to be able to demonstrate maneuvers. The second set of control is also provides a safety factor in the case of an emergency. With the dual controls, the instructor can take back control of the airplane if he feels that the student is placing the aircraft in danger.
Most of us can understand the second set of controls in the training situation but, why do airliners have them also? Are we paying to fly with student pilots at the controls? The primary reason for having two pilots on an aircraft is to divide the workload. One pilot may be working the radios while the other one flies the airplane. It is easier to use a checklist with one pilot reading the items and the other pilot performing the required action. Additionally, the two pilot system allows the pairing of senior and junior pilots. The junior pilot receives time at the controls under the supervision of the senior pilot. This situation allows both pilots to relax for a portion of the flight while the other is at the controls, provides backup in case of an emergency with one of the pilots and allows the senior pilot to take over in the case of an emergency.
Increasing the complexity of any system adds to both the cost and increases the chance of failure. Providing dual controls in aircraft offsets both of these disadvantages by providing a better training platform and a method for splitting pilot workload while providing experience to junior pilots.