When making a Lone Star quilt, I had take in a few short sections of seams to get intersections to match exactly. Typically, the new seam would be only about 1 mm over from the original. I tied off the ends of the new seam and left the old seam alone. (I tapered both ends of the new seam into the old seam.) I ironed my seams open, and I was still happy with how flat it was. (I suppose that if I looked at just right spots there are very small areas with more than 2 layers of fabric, but when it's only 1 mm wide, it is hard to notice.)