How far do you go? do you allow them to parry or evade what was essentially their own attack? I say no. One attack, one reflection (with NO modifiers or chance to avoid).
In the novels this rune was used once by an eastern sorcerer to reflect arrows and not melee attacks. Perhaps it should be limited to ranged attacks, but in the end we removed it from our campaign completely.
With melee weapons it's easier to justify that it's a complete surprise, but if arrow flies 150 meters and then turns back it's possible to see that and be aware of the attack.
The runes wouldn't actually oppose each other, they would just reflect attacks until one of them ran out of oomph if I decided to rule it that way.
The arrow (or other weapon) is affected by the powers of the two runes and effects are exactly opposite. In that case my ruling would be that the rune with stronger magnitude wins. If runes are of equal power I might even make the arrow stop in mid air and fall down or alternatively reflections of same strength neutralize effects of each other and arrow continues to it's original target. Ping-ponging occurs only if you interpret that it's a new attack when arrow is reflected, while it could also be considered as a single attack (made with single CA) which is directed to the target which has the weakest magical defense (although it still could look like ping-ponging or arrow stops in mid air, stays there for a moment and then goes to the direction of the weakest defense.)
The whole thing is open to interpretation and I'm not saying that any of my thoughts are correct, but these are just some ideas.