Double Damage Question

The option to damage twice as much as 5% of the item's options, and if you have 3 5% twice as much damage, do you have a 15% chance by adding up?
Or is it possible to duplicate each other with a 5% chance? 5% chance to damage twice as much as 3 times more damage?

15%
And it's just either double dmg or normal. Doesn't go triple