I don’t often run into this situation, but when I do it is a chore to fix.
Lets say I have a four separate lines drawn like a square. If I duplicate one of those lines and join the five lines, I will have a self intersecting curve. How do I fix this curve? Is there a way to highlight the intersecting segments so I can go in and delete them? My situations are not usually this simple- the shapes are more complex and I only find out that the lines are self intersecting when I see the pop-up window as I attempt to extrude the curve.