I am currently working with data that has a huge amount of topological errors. These are lines that are closed, in the place where the beginning of the line meets the end, someone added more than 2 vertexes. That causes all problems. There are too many features to correct them manually. I've tried clean, but it doesn't work as I expected. Is there any way to remove these overlaping vertices in lines and correct topology in QGIS or GRASS?
In the Processing Toolbox, search for "Remove duplicate vertices". There's an inbuilt tool which takes a tolerance and will strip out any duplicate vertices within that distance tolerance from other vertices.