Once upon a time:

Manually click on each person in a desired twitter list and add them one-by-one to a new one you created. Follow with desired edits.

Now:

Select the list you want to copy. Click a button.

And, keep in mind you still have the option to filter the list (by location, bio) or edit it further manually… if you want :)

IMPORTANT: If you plan to clone a list you expect to be deleted do not forget to turn off automatic updates! Otherwise the list will update to whatever it is a clone of which if it is deleted will be nothing.