When did you last test? For me, using Firefox 59, there's a "Duplicate Tab" entry when I right-click on a tab.

Also, I know for a fact that you've been able to Ctrl+drag to duplicate a tab in Firefox for years and years now, because I do it every week. :)

If you have a mouse with a middle button (or just one with a left button and a keyboard with a CTRL key) you've been able to do this since probably Firefox 1.0 by clicking thusly on the refresh button in the target tab.

I believe Mozilla added it back in with the Photon work.

or middle-click the reload button?