function duplicateKeyError(sKey) {
		return JSON.stringify({
			error : {
				code : "409",
				message : "There is already a user with user name '" + sKey + "'.",
				target : "UserName"
			}
		});
	}
Returns a proper HTTP response body for "duplicate key" errors
@param {string} sKey - the duplicate key
@returns {string} the proper response body