Currently, there is no functionality to duplicate an Action/Matter within Actionstep.
You could use the 'Related Action' feature to create a new action which as part of that feature you can choose to copy the Action/Matter Participants and Custom Data from the source Action to the destination Action.
This does not duplicate any Documents, Eemails, Tasks or File Notes. Only Participants (Contacts) and Custom data can be duplicated from source to destination via the related Action.
Please contact your consultant to learn more about Related Actions.