Doublers are SPECIFICALLY made as 0 (zero) nic juices so that you can buy them to 'water down' higher nic flavors of the exact same kind.
Example;
I have 36mg nic Butter rum, 30 ml
I buy also 0 nic Butter run doubler, 30 ml
I mix the 2 (1 for 1) and have 60 ml of 18mg nic
juice
The taste of the juice is NOT impaired as I have used the same base flavors - only the nic level is altered/reduced.