Go to the amcatalog, "all" tab. Localize the part if it isn't already by right clicking on the name then "localize". Now you can right click on that localized part and choose "copy definition". Give it the new part name. These parts can be externalized again but be aware that if they have drawing views associated with them, the views will be lost. To keep both parts external, open the part you want to copy, save it as the new part name, and insert that new part into your assembly through the catalog.