Grayhawk said:
Now, should this AoO also be for double damage?
I don't think so. It is, as you correctly pointed out, just a normal AoO, not one granted by readying a weapon, or by the feat that mimics readying the weapon.
If someone charges a longspear wielder who has the Instant Ready feat, and assuming the longspear wielder has Combat Reflexes (since otherwise, he only gets one AoO per round), I believe the course of should be thus:
1) Foe charges.
2) Character gains AoO from Instant Ready, doing double damage against the charge; foe is at 10 feet at this point.
3) Foe moves 5 feet closer, continuing the charge.
4) Character gains AoO from foe passing through threatened area, doing normal damage, because it's a normal AoO.
5) Foe attacks.
Of course, if the attacker is using a lance or other reach weapon, steps 3 and 4 don't happen.