What's a doubler? A doubler bottle contains double-strength juice concentrates for vapers who want to add their own nicotine. Each doubler bottle is twice the volume of the requested juice to make it easier to mix. For instance, a 15ml Doubler comes in a 30ml bottle. Singler bottles contain pre-diluted juice concentrates and are for vapers who don't intend to add nicotine to their juice. For more information, read our guide to juice mixing.