I do own multiple copies of the same (cheap) light - all of them being bought with a specific purpose and all of them being used in different places.
I don't have any lights where I have bought a spare purely as a backup to stay at home in the box until I need it.
Generally with lights (and indeed with most technology) I have found that the technology advances as quickly enough that when the time comes to buy a new light (or camera, phone, etc) that I will be able to get something newer and better than what I am replacing. To have bought two would just mean I am stuck with the older technology for longer when I had no real need to. That is of course assuming that I do have other lights, so I am never completely without backup or alternative...
At the same time, I do always reserve the right to change my mind on the subject - if I find a light that I particularly like for certain reasons that I don't see in newer/alternative models, I may be tempted to pick up spares, but that is something I have not done yet.