Select a Tag from the Tag Browser and right-click. In the context menu add an item for 'Duplicate Tag'.* This will open the Tag Editor window with a duplicate of the selected tag except for the name. Don't create the tag yet! Allow the user to edit the tags properties and it's name first. Create the tag when he saves it. For extra credit, refresh the tag browser window and highlight/pre-select the tag he just saved so he can just right-click again to create yet another tag with same general properties without hunting through the tag tree.

* there is already a 'duplicate' item in the Edit drop down menu but it does not do anything when a tag is selected.