In a 3D model, coincident or coplanar faces are two or more faces or polygons that occupy the same space and represent the same surface. These types of faces cause rendering problems, particularly a distracting flickering effect in animation renderings. Importing a CAD model into a 3D application can cause coincident faces, as can some face modeling techniques.
Coincident/coplanar faces cause a problem when (a) the face normals of both faces point in the same direction and (b) only one of the faces is needed. Because the faces are in the same place, the renderer has trouble figuring out which one is "in front". Thus, the renderer renders one face on one frame and the other face on the next frame, causing flickering in animations.
Coincident faces are distinct from overlapping faces, where the faces of two adjacent objects or elements touch and are coincident with one another. An example of overlapping faces is legs under a tabletop. The top faces of the legs might be exactly coincident with the bottom face of the tabletop, but the face normals point in opposite directions and the overlapped area is not visible in renderings. Overlapping faces do not cause problems in rendering.
This phenomenon is also distinct from intersecting faces, where the faces of an object pass through other objects' faces at an angle. Such geometry causes no problems in rendering.
For clean geometry, you should detect and fix any coincident/coplanar faces before publishing your model for sale.