On the one hand, the damage isn't called out in any sort of specified way. It's not common for damage to be, but with buffs, usually a type is applied to prevent overstacking it (for example, Bear's Endurance adds an enhancement bonus to Constitution).
On the other, it's still from the same source, and the way spells usually work is overriding duration, not adding the effects together.
I would rule as a GM that it overwrites the previous spell. However, if the player fought it, I might allow to pick the best 1d6 rolled per casting to last for the most recent cast.
It has been a while, but nothing in the description says it doesn't stack. I think it is unnamed so it would stack.
Compare it with http://www.d20pfsrd.com/magic/all-spells/r/ray-of-enfeeblement or any other ray, in that description one can read:
"This penalty does not stack with itself. Apply the highest penalty instead."
What do you think?
Bonus Types: Usually, a bonus has a type that indicates how the spell grants the bonus. The important aspect of bonus types is that two bonuses of the same type don't generally stack. With the exception of dodge bonuses, most circumstance bonuses, and racial bonuses, only the better bonus of a given type works (see Combining Magical Effects). The same principle applies to penalties—a character taking two or more penalties of the same type applies only the worst one, although most penalties have no type and thus always stack. Bonuses without a type always stack, unless they are from the same source.
It has no type, so it stacks.
wraithstrike wrote:Penalties from the same source don't stack. Even rikkan's quote says that. He just overlooked it. You take the worse of the two.
Uhhmmm no?"although most penalties have no type and thus always stack."
Penalties follow the same rule as bonuses and same type bonuses don't stack even untyped bonuses if they are from the same source.
Bonuses without a type always stack, unless they are from the same source
Stacking Effects: Spells that provide bonuses or penalties on attack rolls, damage rolls, saving throws, and other attributes usually do not stack with themselves.
Therefore the spell does not stack with itself. It would need a specific rules exception to do so.