v.out.ogr should issue a warning when exporting a geometry with multiple categories

[marisn]

When v.out.ogr exports a feature consisting of a single geometry and multiple CATs in a single layer (such thing is legal in GRASS) to format that doesn't support such feature (ESRI Shapefile), feature geometries are duplicated to create "single feature = single CAT + attributes". Such action preserves all data but introduces duplicated geometries in output data set. In such case v.out.ogr should print a warning about duplicating geometry in output data set as it might break v.clean rmdupl results and produce unexpected output.