There are two ways for you copy a Twitter List. You can find a Twitter List and fork it by clicking the Fork menu on that list, or you can start a new list and import all the members. Both will have the same effect at the end, however if you fork the list, both lists will be connected on Listpedia and in the future you'll be able to better manage your lists by getting account suggestions of new accounts to add to your list.
Before you go out and create a copy of a list, consider that you only need to create a copy of a list if you want to preserve its current member list -- in case the list owner decides to change things or delete the list -- or if you want to modify the list. An alternative to creating a copy of a Twitter List is to simply subscribe to the list, or you might be interested in exporting the list to a CSV.
NOTE: Listpedia doesn't allow you to copy private lists and only the first 500 accounts will be copied.
Forking a list is the equivalent of taking a snapshot in time from the other list. You can fork a Twitter List and create another Twitter List.
These are the steps:
With this method you are actually importing the members of a Twitter List into a new list. This can also be used to create a new list from the list of Friends of Followers of an account.