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?