From experience I can tell you that over forming double flairs will keep them from sealing.
Nickel copper is easier to form than steel, so if you really clamp down on the second flair there is no give left for it to conform to the fitting (unlike a single flair that will conform to anything).
When you make the double flair in nickel copper, form the first stage nice and tight then stop a little short on forming the second flair. The nut will pull it in and seal easily.
In my case a new hydraulic flairing tool made it very easy to squeeze the second stage too much.
