The Actions panel menu has Duplicate, between New Set… and Delete. With
an action selected it copies that action to right after it in the same
set; otherwise it copies the current set, with all its actions, to right
after that set. The copy is named after the original with " copy" added
and is selected, and a copied set gets an id of its own. Copies are
saved with the other sets.
ui.actionSet.duplicate {set?: index, action?: index} does the copy and
returns {set, action?}. An index past the last set or action is an
error. A recording into a later set and an open Set Options or Batch
dialog stay on their own set and action.