We often need to delete duplicate surfaces but I can't get what is a duplicate surface.
Duplicate surfaces are surfaces that share exactly the same position in space (we would get duplicate surface by copying). They should be removed before meshing by Geom>defeature>duplicates.
Does it means a new surface is *overlapping* upon former.
yes
How 'copying error' is generated.
Duplicate surfaces are surfaces that share exactly the same position in space (we would get duplicate surface by copying). They should be removed before meshing by Geom>defeature>duplicates.