My players are about to fight Marsh Giants, which have the supernatural ability to cast Bestow Curse. I found this answer for 3.5, I wasn't sure if the same rules apply here.
Can I curse my players into oblivion?
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The rules for combining magic spell effects read:
Stacking Effects: Spells that provide bonuses or penalties on attack rolls, damage rolls, saving throws, and other attributes usually do not stack with themselves. More generally, two bonuses of the same type don't stack even if they come from different spells (or from effects other than spells; see Bonus Types, above).
Same Effect with Differing Results: The same spell can sometimes produce varying effects if applied to the same recipient more than once. Usually the last spell in the series trumps the others. None of the previous spells are actually removed or dispelled, but their effects become irrelevant while the final spell in the series lasts.
One Effect Makes Another Irrelevant: Sometimes, one spell can render a later spell irrelevant. Both spells are still active, but one has rendered the other useless in some fashion.
You can stack the same spell multiple times as long as it is doing different things. So you could curse them 6 times and reduce all of their ability scores by 6 once but not 6 times and reduce one ability score by 36.
The Remove Curse description implicates thatImplicating that a creature can be affected by more than one curse.
Afflictions, such as curses, may or may not stack. Ability damages are cumulative, penalities are sometimes cumulative.
Effect: This is the effect that the character suffers each time if he fails his saving throw against the affliction. Most afflictions cause ability damage or hit point damage. These effects are cumulative, but they can be cured normally. Other afflictions cause the creature to take penalties or other effects. These effects are sometimes cumulative, with the rest only affecting the creature if it failed its most recent save.
Penalties from spells with the same descriptor don't stack, at least for characters.
So yes, you are allowed to curse them (almost) into oblivion but, if the came prepared, they have tools to get back in the game quickly.