I have mistakingly overlapped a little before with no breakage. You just need to be very careful. Just don't comb the relaxer purposely over the already relaxed ends.You may want to use some sort of protectant on the previously relaxed hair. I use Creme of Nature Scalp protector(can use it one hair too). This may give you some peace of mind.
Make sure to follow up with a good conditioning treatment. I use Nexxus Keraphix, it is a reconstructing protein treatment.. There are others though. I also follow up with a moisturizing shampoo.