It's a product offered by tastyvapor.us
It consists of 2 or 4oz of zero-nicotine liquid, but is otherwise identical (in flavor) to any of their "regular" eliquids.
A doubler is used to cut down high-nicotine liquids without having to diminish flavor.
For example, I usually order (2) one-ounce bottles of 48mg, 10%VG juice in a particular flavor (for example, Butterscotch Cheesecake). Then, I order a 4oz doubler of Butterscotch Cheesecake @ 70%VG.
Finally, I mix 5mL of the nic juice with 10mL of the doubler to create a 15mL bottle of 16mg, 50%VG juice.
The cost for that order would be $45 for 180mL of gourmet liquid (unless the flavor is one of the weekly specials, in which case it would cost $35).
Enjoy.