Coming back to mechanical locking devices for threaded assemblies, double nut is probably the most common one. Wherever the assembly design allows it, it’s the simplest thing to do: just add one more nut of the same size you already have anyway for the bolt in question.
While the second nut does increase the surface of interfaced threads, it doesn’t eliminate the gaps between them. Certainly, you can argue that it will take that much longer for the vibrational impact to shake such a bolt into loosening, but the loosening will still inevitably happen.
Additional nut increases one more thing: it doubles the amount of surface that can corrode together between the bolt and the nuts. And that’s a nasty surprise that you normally discover when you’re about to dismantle the piece.
Finally, out of all the mechanical locking devices, the nuts are the heaviest, and if you use several double nuts to secure your bolts on a piece of equipment, you definitely have to count with all the additional weight you’ve added to it. As far as elegant solutions go, this is not one of them.
If you replace the second nut by just a few drops of a Loctite Threadlocker, you’ve not only found a much more elegant solution, but you’ve really resolved the issue of vibrational loosening – permanently.