Anybody who watches football knows that Double Coverage is where two players are used to defend against one pass receiver. In basketball it's called double teaming. In medicine double coverage refers to using two different antiobiotics to treat one infection, increasing the odds that at least one of them will work. In medical insurance double coverage is where two people, typically a married couple, each have employer sponsored plans covering both of them. In car insurance it refers to two policies covering the same thing, typically when a sleazy car rental company sells you extra insurance that you're already covered for in your main policy. In geocaching, well, I don't know what it is.