The circumstances in which Lily sacrificed herself for Harry were unique. Five characteristics in particular seem important:
- Lily loved Harry.
- Lily did not try to protect Harry by fighting.
- Harry was defenseless; Lily was his only remaining protector.
- Voldemort had a specific intention not to kill Lily.
- Lily had no idea that her sacrifice could cause the Curse to rebound.
Dumbledore attributed Harry's survival to love. And, although Harry did not know it until after Dumbledore's death, there were two loves involved--Lily's love for Harry, and Snape's love for Lily. Voldemort would have intended to kill Lily without Snape's request that her life be spared. Had he not made that request, Lily's sacrifice would not have been effective, because she would have been on Voldemort's hit list. Circumstances like this are very rare. For a witch or wizard not to at least attempt to fight back is, of course, very unusual. Even more unusual is for a murderer to specifically intend to spare one of his victims' relatives.
While it might be possible for a Killing Curse to have rebounded before, the Curse aimed at Harry was apparently the first to rebound within recorded history. Voldemort, for all his knowledge of Dark Magic, had no idea that a Killing Curse could ever go wrong. Neither did Lily. Most likely, if she had known, then the Curse would not have rebounded: later on, Dumbledore did not want Harry to know he might survive Voldemort's Killing Curse again, because then Harry's attempted sacrifice would have failed. Harry had to believe that he was genuinely going to die for people he cared about. That was what his mother had done. In both cases, the sacrifice worked, and the people who needed to be protected were saved. (And Harry survived his sacrifice thanks to several even more unusual things, which involved his mother's sacrifice and his own status as Voldemort's final Horcrux.)
In short: It requires a very complicated set of circumstances for a Killing Curse to rebound. Love is the most important factor, but several other things are also required. Such a situation cannot be set up intentionally, or the Killing Curse will function as usual.