By default, the player has a 0% chance to receive double loot. All equipped items and other effects that increase the double loot chance are added together to get the player's total chance to receive double loot.
For example, if the player has one item equipped that gives a +10% chance to receive double loot and another that gives a +5% chance to receive double loot, their total chance to receive double loot is 15%.
Combat
Any item received as loot from combat has a chance of being doubled, including bones and chests from completing Dungeons. The roll is made once per drop - Bone drops can be doubled without the item drop being doubled, and the opposite is also true. If the drop would consistent of a stack of multiple items, one roll will be made to determine whether or not the entire stack is doubled.