I've noticed that my efforts often accumulate troublesome bits of lines and polylines, even full duplicates, hidden under the visible ones.
So, what might I be doing to create these inadvertent under-lays, so I can minimise them happening?
It was some time before I realised they caused many of my polyline, hatching and copying difficulties, but now know to use the Select pop-up list and its highlighting to find and remove them. I think that although a tool's own Select highlighting may appear as expected, that's to help me, not it; but hides the fact that the tool has actually picked up a fragment so can't detect the next intact, visible entity. Delete the bits and assuming no other errors like subtle line-breaks, the tool works as it should.
At first I could not understand why the last line does not over-write the earlier, then realised such "data-stacking" might be vital to many important drawing functions working or even existing. It even occurred to me that TurboCAD uses a formal version of the effect for Layers - that's a guess, I don't know!