While I tend towards minimalism in my equipment, there are various instances where it seems smart have some redundancy. In the photo above the light on the left is actually 2 Watt Blaze head attached to a 1/2 Watt Blaze body. My old, larger handlebar bag bounced against the light on a bumpy trail, sending it flying. The body of the light cracked, but I was able to duct tape things back together well enough to get home. I now run with two lights, mounted better. I carry spare batteries anyway, why not carry them in a spare light?
Note the time on the two computers, nicely illustrating the old proverb that "a man with one watch knows what time it is, while the man with two watches is never sure." The Tour Divide maps wisely state "A mileage odometer is absolutely necessary to enable you to follow the narratives. DO NOT attempt to ride this route without an accurately calibrated odometer." While many Tour Dividers choose to rely on GPS, I rely on redundant computers. Both use CR2032 batteries and both are pretty simple, wired models. The Cateye's sensor reads off the front wheel, while the Bontrager reads off the rear. I have them mounted on the top-tube, where they are reasonably well-protected and out of the way yet visible.
For some things, like frames,bottom brackets, and your own body, you bet it all on one thing that should be tough enough to go the distance. For some other things, a back-up unit is worth the extra weight.
Keep 'em rolling,
Kent "Mountain Turtle" Peterson
Issaquah WA USA