How to light back side of quad mesh same way as front side?

Normally, when lighting effect applied to quad/plane(or any paper) mesh, one side is bright and the other side becomes darker. But I want both sides of the mesh to be lit as if both sides are facing light source to make them brighter for some game scene.

I assume this is shader’s work but I’ve never written one so have no idea where to start. Could anyone give me some hint? Thanks.

[Edit]
When I tried Cull Off shader with quad/plane mesh, it did give both sides of the mesh same lighting but when the front-side is not lit, neither does the back-side.

Just use a single-sided quad and put “cull off” in the shader. Then both sides will be rendered with the same lighting.