Preppers often say, you don't have it, unless you have two of it, and you don't have a backup until you have three.

They all say that so that people overstock useful items, so when nuclear zombie virus apocalypse hits and normals all die, there's lot of supplies stockpiled and easily accessible for the survivors.

s/

Sometimes it is better to have one, just a very good one. Depends on the budget.

That's a maxim for computer data backups as well.