Creating a classic duotone effect in Lightroom is super-simple. 

Step 1 - Head over to the HSL/Color Panel, and select the Saturation option. As illustrated here.

Step 2 - Desaturate all of the colors except the one your want to keep. In this case, I left red saturated.

TOP TIP -If there are other areas of the image that are still saturated with the duotone effect that you don't want. Head over to the Adjustment Brush and create a brush with -100 saturation. Then paint over the offending areas.

TOP TIP - Desaturating the other colors may leave your image a little flat. You can fix this using the Contrast slider in the Basic panel.