Currently if you call prepareUnique: true on a unique index, the index spec will still be changed, which is kind of confusing:

{
		"v" : 2,
		"key" : {
			"a" : 1
		},
		"name" : "a_unique",
		"unique" : true,
		"prepareUnique" : true
} 

It seems prepareUnique: true should just be a noop on a unique index, and not change the index spec.