If one character used the Avada Kedavra against an armed wizard (has his wand on the ready), can that armed wizard use the same curse to block Avada Kedavra? I know Avada Kedavra is considered "unblockable", but assuming that it is the same spell, I wonder if both curses would just nullify each other, if they are cast will the same amount of intent and power.
Unforgivable Curses have to be cast with a desire to make pain, control, kill, as we see in OOTP:
“Never used an Unforgivable Curse before, have you, boy?” she yelled. She had abandoned her baby voice now. “You need to mean them, Potter! You need to really want to cause pain — to enjoy it — righteous anger won’t hurt me for long — I’ll show you how it is done, shall I? I’ll give you a lesson —”
-Harry Potter and the Order of Phoenix, chapter 36
If you're casting it with the intention of blocking, it probably won't work.
This is in addition to the fact that it would be nearly impossible to hit the spell on purpose...
The spells would most likely ricochet following precedent.
Spells hitting head on tend to ricochet
Jets of light shot from both wands, hit each other in midair, and ricocheted off at angles — Harry’s hit Goyle in the face, and Malfoy’s hit Hermione.
but what about a spell ricocheting Avada Kedavra you ask?
yelled his best hope to the heavens, pointing Draco’s wand:
The bang was like a cannon-blast and the golden flames that erupted between them, at the dead centre of the circle they had been treading, marked the point where the spells collided. Harry saw Voldemort’s green jet meet his own spell, saw the Elder Wand fly high, dark against the sunrise, spinning across the enchanted ceiling like the head of Nagini, spinning through the air towards the master it would not kill, who had come to take full possession of it at last. And Harry, with the unerring skill of the Seeker, caught the wand in his free hand as Voldemort fell back- wards, arms splayed, the slit pupils of the scarlet eyes rolling upwards. Tom Riddle hit the floor with a mundane finality, his body feeble and shrunken, the white hands empty, the snake-like face vacant and unknowing. Voldemort was dead, killed by his own rebounding curse, and Harry stood with two wands in his hand, staring down at his enemy’s shell.
“I miscalculated, my friends, I admit it. My curse was deflected by the woman’s foolish sacrifice, and it rebounded upon myself. Aaah . . . pain beyond pain, my friends; nothing could have pre- pared me for it.
Avada Kedavra has precedent of being able to be richocheted.
Only if both wands share the same core
When a duel occurred between wizards with wand cores harvested from the same creature, simultaneous spell-casting by both parties would trigger a rare, powerful effect called Priori Incantatem.
If both wands share the same core, then two wizards casting Avada Kedavra at the same time would induce Priori Incantatem and lock the two wizards together until the spell ran its course.
Of course, if the two wands didn't have the same core and the two wizards (assuming both had intent to kill) cast Avada Kedavra at the same time, it would simply result in the deaths of both wizards.
The only known protections against avada kadavra are
- Avoiding being hit by dodging or using a physical object
- sacrificial love, it is not known if this protects you from the spell, or just the person who killed the sacrificing party.
- Sharing a core and forcing them to battle, can result in priori imcantartum.
- your opponent using a wand that owes its allegence to you allowing it to rebound.