It's not to do with having two Sarahs, it's either you edited the person you were adding her to or you had the window open too long and it timed out, causing a conflict error.
If you have infant deaths, you can tick the "no children" box in edit mode and you can go into "add spouse" and there is a "no spouse" button. These will help distinguish the two as well as a note in the bio and birth/death sources added.