I can manually remove them by deleting the surrounding faces and remaking them, but there's got to be a faster way.
I get these lines after joining Ctrl + J two separate objects (e.g. two mirrored halves of the body via Duplicate + Scale by -x) and they don't always appear at the fusion points. I'd thought they meant duplicated vertices, but merging them M at center doesn't do anything. Could it be a duplicated edge, though?
If it means anything, the horizontal thick-line was a former UV seam.