While it would be nice to have this added directly into Safari, you can use the following key combination to duplicate tabs currently:
- Open/Visit the tab you want to duplicate
- Hit the combos: Command + L and then Command + Return (or Enter)
The first combo selects the location bar in Safari, whose default behaviour is to select the fully available link. The second combo asks Safari to open the selected link in a new tab. Together, these help you duplicate an open tab’s link into another new tab.