I noticed that this occurred in the previous version, but decided to wait until this new version 2 was released before reporting in case it was fixed. Alas, it is not.
Upon pressing the list all tabs button at the top of my Firefox window, I get a normal display of the opened tabs and a sub-menu item for container tabs, as so:
However, if I then mouse over the area where it says New container tab it duplicates all of the opened tabs showing in the list, as so:
If I move the mouse off the New container tab menu item, it then reverts to showing none of the opened tab entries, as so:
This is reproducible for me 100% of the time.