I wouldn't duplicate it. There's a whole enrollment thing with SMGR and security certificates and FQDNs and stuff - so you couldn't even try if you wanted to unless you took the primary offline.
And then, the backup whenever you'd use it would still be out of date and SMGR would see it like some impostor.
So, you gotta build a new one out with new hosts/IPs etc.
The config involved is only in how detailed your SIP design is. Do you have AAM? Need to point all their trunk servers to both SMs now. Same for any other adjunct - CM included - you'd need to map out all those routes to include the new sig/trunk to the new SM you're building.
As far as licensing goes, you should be fine, though it does depend on the release you're at. It ultimately is just based on sessions at 6.3. Its instance based at 7.0 - so you might need to pay for it if you're on that load. Although, the "right" way if at all possible is to have the single sw instance virtualized in VMWare appropriately so VMWare licensing to move it across servers makes your life easier by only worrying about the single software instance - and you got less trunks to play with on top of that.