What is a double-sided material?
Suppose you want to create a box in Blender and you want to remove one of the faces to show the inside of the box. Although in Blender the cube looks all right (we can see the inner part of the box) in Game Mode, the box looks a bit strange. See the screenshot below:
In order to repair it you have to set the material to be double-sided.
How to
If you plan to set a material to be double-sided, do as follows: select your model and open the Material Panel. Scroll down to the Custom Properties section and click Add button. It will create a new custom property with a default name "prop". Click edit button and the Edit Property window will appear. Change Property Name to DBL and close the window by clicking OK button:
The value of DBL custom property should remain default (1.000), although it's not taken into account by Dark Exporter.
How to debug double-sided materials?
Dark Exporter reports double-sided materials in Blender Console. On the below example you can see the material named Bulb has been detected to have DBL custom property: