Tons of people die by this spell, the premise of the books is that Harry is the one person to survive it, ever. This is explained by saying his mother loved him so much that he was shielded. I can't imagine no other wizard ever has been in the same situation. Voldemort killed many many people and the situation in the Potter house, a mother refusing to give up her son, must have been something he saw quite often.
What was so special about this situation that caused the spell to backfire, because if simply loving your son (a lot) is enough to shield him from death a lot more people should've survived. Even if wanting to sacrifice yourself for your son (like Lily) is required, that must've happened before...
I don't think the linked question's answers answer my question. I'm assuming the exact same situation as with Lily and Harry must've happened before with other people. There has to be something more that sets it apart, simply wilfully protecting someone until the end even though you can choose to step aside is not a unique situation, think about two lovers being threatened by Avada Kedavra.
Edit: I understand that it's the sacrifice that triggered the protection, not the dying. My point is that Lily responded like any mother would and that death-eaters are the types of people who like to give mothers the chance to give up on their son to stay alive.