I’m 22 year old guy and think about buying Instant Shine double strenght. Is this a good idea? I know that it’s okay to buy 2x socials like IO and IH but what about IS?
That should be good, ISx2 is what I have and I have not had any problems with it.
It will be fine - You rarely use just one spray of IS! 
I’ve actually just ordered 2x IS. When I wear regular strength, I usually wear 3 or 4 sprays.
after my initial to-go, all my orders of IS were for the double strength. Earlier today I went a little bit bolder and ordered a 4x 
that was the result of my recent return to using this brilliant product after a long break (from the IS, I kept using other mixes) and realising how good it is.