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%.
Any item received as loot from combat has a chance of being doubled, including bones and chests, Fire Cape, Infernal Cape, and Infernal Core 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 consist of a stack of multiple items, one roll will be made to determine whether or not the entire stack is doubled.
The additional chance to double items in Thieving is based on stealth and NPC perception, double items can therefore be guaranteed from stealth alone if the player's stealth is at least four times the NPC's perception.
Thieving.
Mastery Tokens, 
Mysterious Stones, 
Circlet of Rhaelyx, 
Jewel of Rhaelyx, 
Signet Ring Half (a), or 
Signet Ring Half (b).
Fire Cape, 
Infernal Cape, and 
Infernal Core 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 consist of a stack of multiple items, one roll will be made to determine whether or not the entire stack is doubled.
Bobby's Pocket, 
Chapeau Noir, 
Sneak-Ers, and Area/NPC Unique drops.
Combat:
