Doublers are designed for users to add their own nicotine to their e juice, especially lower strength base nicotine. The bottles are sold half-filled with double the concentration of flavouring so that when nicotine is added to the bottle then e-liquid is diluted to achieve the flavour you love at the strength you need.
Eliquids in this range are not suitable for vaping without dilution. If you don’t add nicotine, sticking to ‘normal’ juice (singlers) is your best bet!
Doublers and singlers look the same; however the doubler will be filled only half way whereas the singler will be filled to the top.