As the name suggests, Double Flavour has double the flavour concentrate and is designed for mixing 1:1 with unflavoured Nicotine.
Nicotine is available from a number of overseas suppliers, in various strengths, eg: 24mg
If you purchase 36mg Nicotine and mix 10ml of that with 10ml of Double Flavour you will have 20ml of full flavoured E-Liquid at 18mg
The Flavour Republic double flavours are produced in a PG base so that means you will need to purchase your nicotine in a VG base to achieve a good mix of flavour and vapour in your E-Liquid
In this example you will put 5ml of Cheesecake Double Flavour and top up with 5ml of Nicotine (36mg VG Base). After shaking you will achieve a 10ml bottle of Cheesecake at 18mg!