which v.clean option is required to eliminate overlapping segments of lines?

Question

I want to produce a polygon layer out of lines like in this image with the command poligonize:

Source: QGIS create boundaries from background image

The problem is that some of the lines are coming from polygons with the command polygon to line tool in QGIS like in this image:

Source: How to merge overlapping and duplicate lines from vector layer to one single line?

I am struggling to understand which options I should use in v.clean to remove the overlapping segments of the line layer, or which other stepts or tools I could use to get the desired output in a straight way.

The main asumption is that lines match exactly.

My software QGIS 2.18 and GRASS 7.4

Answer 1

I couldn't get v.clean to remove duplicates in the way I was expecting using rmdupl (or the -c option*). However, the QGIS algorithm "Delete duplicate geometries" works well. Steps:

• Break lines at intersections with v.clean's "break";

• Remove duplicates with "Delete duplicate geometries".

• Create the new polygons with "Polygonize"

These could be chained with a single Processing model (click for gist of a model file).

*The 'recommended followup tools' in the dialog adds the -c option. From the help: Hint: Breaking lines should be followed by removing duplicates, e.g. v.clean ... tool=break,rmdupl. If the -c flag is used with v.clean ... tool=break, duplicates are automatically removed. Not working in this case for some reason.