At DHR we often get questions on whether or not a ship needs Duplex navigation lights.
The regulations according duplex lighting are prescribed in Marine Safety Committee resolution MSC 253 (83) . 
The resolution states that the masthead lights, sidelights and stern light of a ship not less than 50m in length should be duplicated or fitted with duplex lamps.

Although this blog is written to explain duplex lantern,  the resolution also states that ships not less than 50m in length are fitted with a navigation light controller (NLC). So we have added that part of the resolution as well, because if you are obliged to use duplex lanterns, this is also mandatory.

The NLC should provide an alarm for:
a. failure of the power supply to navigation lights.
b. failure, short circuit included, of a lamp that is switched on.

The NLC should display the status of the navigation lights in a logical presentation

Dredgers, working Boats and the practical use of Duplex navigation lights.
A dredger must display two all-round green lights on one side and two all-round red lights on the obstructed side of the vessel. These lights need to be on the same horizontal line.
Two duplex lanterns, red over green, are used on one side and two duplex lanterns, green over red, are used on the other side. This makes it  very easy to indicate the obstructed side and the side that is clear to pass. On inland navigation vessels, white over red and red over white duplex lanterns are used to indicate a ship at work.

Besides the rules for duplex lanterns, there is also the practical  use of the duplex lanterns.
Take for example a tugboat smaller than 50m in length. When engaged in a tug or tow at night, a navigation light can fail. However, because of the nature of work, it is impossible to change the bulb.In these situations it is very convenient to have duplex lanterns and remain visible during the job.