This seems like it should be easy, but I'm not figuring it out: How do I convert a polygon layer into a line layer representing the polygon edges, where each edge belongs only to a single feature (or part)? When I convert from polygons to lines, each edge is represented twice (because each edge belongs to two adjacent polygons). Even if I dissolve the features, the duplicates remain as separate parts, so I cannot just dissolve, then split into singleparts. This causes problems with symbolization, for example if I want to use a dashed line style.
…and then to remove duplicates, for example using v.clean with rmdupl. But I can't figure out how to do either of those steps.
For splitting segments, either the answer isn't in those two linked threads, or else I'm not understanding the explanation. Using v.split doesn't work, because I can only select to split by length, or by number of vertices, not by intersections with other line segments.
For removing duplicates, rmdupl isn't working; the duplicate edges always remain after I run the algorithm. It seems that the edges aren't recognized as duplicate geometries, even though they have identical points. (Running snap first didn't help; the layer appears to be topologically correct already.)
What am I missing?