For some reason I think that most ladies end up overlapping a tiny bit while relaxing. It seems that it may be kinda hard to only apply the relaxer to the roots only without even touching previous relaxed hair (below the demarcation line [sp?]....the part of the hair strand where the newgrowth meets the relaxed part.). Me, I coat my previous relaxed hair as much as I can with conditioner and petroleum. i try to get as close to the "line of demarcation" as possible without covering up any of my newgrowth. I don't purposely overlap, I only touch-up my roots, but I'm also realistic when thinking that maybe a centimeter of my relaxed hair from the demarcation line has been overlapped. You have to understand that you have to be quick when applying a relaxer or else you'd be severely overprocessed. To self relaxers, if you take the time to carefully and precisely apply the relaxer to only the roots and not past the demarcation line, then too much time will pass and your hair might become over processed.
These are just my thoughts