I applied the GRASS function
v.clean -> break as suggested by @ahmadhanb and the shapefile now shows correctly all the nested basins without using the
Feature blending mode = Multiply.
However I noticed that in the attribute table the
v.clean function created ~2 times more variables than the original shapefile (e.g. original shapefile 250 polygons, after v.clean 500 polygons). There are then duplicated polygons that eventually create issues when doing a Join.
How can I remove the duplicated polygons created by
v.clean and keep ONLY the 'good' ones that appear in the QGIS environment when selected?
I tried to apply
v.clean but the shapefile did not change at all!