A Duotone is the same halftone image printed twice. Usually a color is printed first followed by black. 

If two identical images were printed in perfect registration, no effect would be seen but, the halftone screen of the second image of a duotone is typically angled 30 degrees. This produces the same image, but with a different pattern of dots.

Duotones are an effective way to enhance photographs when printing in 2 colors. It is possible to print duotones using black only. This results in a very fine image since the spaces between the halftone dots on the first image are partially filled by the second image. This still requires two plates and two impressions on the press.