By definition, Wikipedia describes duotone as:
a halftone reproduction of an image using the superimposition of one contrasting color halftone (traditionally black) over another color halftone. This is most often used to bring out middle tones and highlights of an image. The most common colors used are blue, yellow, browns and reds.
Duotone prints used to take a lot of time and skill to produce, and for many years they were not in among photographers or admirers of photography. The last few years of digital photography combined with advancements in post production have given photographers the ability to create, simulate and replicate many traditional film processes, and duotone images are once again gaining popularity. They are, as said above, most commonly seen in browns and blues, but reds and yellows do make an appearance. This collection of duotone portraits shows how some creative post processing can really amplify and change the overall mood of a subject.