This happened to my sister because she had issues with her first scar in that it was raised and had healed badly. For her second CS they used the same scar line and basically gave her a tummy tuck to try and fix the wound but it didn't work so with her 3rd CS she had to have a second incision about 1.5cm above the first. It's not routinely done and only really when there are issues with the first scar. She also had adhesions from her first two CS's over her bladder and bowel which complicated the 3rd CS and affected the decision by the surgeon over where to cut.