Unless one is shooting commercially there is an argument that the best back-up is a credit card.
Think about how often, if ever, one has a need for back-up. Also think about how few places there are where Canon or Nikon aren't available off the shelf with a wave of a card. Think also of the rental market.
Then think about all that money tied up in back-up equipment that is seldom if ever used.
It’s just a thought.
A very important thought.
I do photography for a living. I do need a backup for when I'm on an assignment as when I sign a contract, I
will, come hell or high water, deliver the goods I've promised.
What I prefer for backup is another identical body and some overlap in lens options. Of course it's senseless (and expensive!) to replicate ad nauseam every piece of equipment and carry the whole truckload all the time, fearing a failure. That's why I usually have a mid-range zoom which I don't use much in addition to the prime lenses that I use most. At the moment I can't justify a second current body (having just purchased the latest top of the line), but I keep my older body which is compatible with all the current ones' accessories and batteries as the backup. The rest of the equipment I might have usually gets sold as cost recovery for the latest purchase these days.
Swapping to the backup ... either the older body or the zoom lens ... in the event of an equipment failure will net a reduction in capability and make me work harder to achieve the results I've contracted to produce, but since I know it can be done I'm comfortable with that.
If you're shooting for yourself (like when I'm working on personal projects), investing heavily in backup equipment seems of little real value. Without a contract to define what I'm shooting and what I must deliver, the photo opportunities for any given project are almost too numerous ...
"yeah, the shutter died in the middle of Thursday's shoot and I lost a hundred exposures I might have made, but there are a thousand more exposures of equal or better value to the project I can make today, two days or a week later..."
I find it better to concentrate, for this kind of work, on ONE camera and lens kit. Learn them well, use them a lot, so you can really forget about them and concentrate on the photography and not the equipment. Have your checkbook or credit card handy when something breaks and you'll recover from the breakage fast enough.