No, I'm sure that others HAVE died for their loved ones. Harry was likely a special case, though - for one, he was an infant. There is NO purer love in this world than that of a mother for her child. Love between adults is a many faceted thing, complex and deep. There are layers, doubts, waxing and waning.
It's like comparing a layer of scales to a thick hide - one has dozens of seams, the other is continuous.
It's also likely that very few wizards have ever tried to use Avada Kevada on infants.
Ergo, it could simply be that this exact situation hadn't arisen previously. Alternatively, it is possible that previously the Dark Wizards involved had recognized the protection given and had someone else strike the killing blow - Big V is frequently said to have a massive blind spot for magic he's not skilled with - he simply dismisses it. If the other person involved was an adult wizard, the loved one's sacrafice may have frequently given them time to strike back, or Apperate away, or any of a dozen other methods to avoid being hit by a killing curse.
In short, this question can only be answered speculatively (unless J.K. has spoken up) but the things that need to be considered are:
truly selfless love is rare
people who will murder a child in its crib are rare
wizards who have just had someone they love die to protect them are unlikely to stand immobile while another curse is fired