Duplicate buttons are displayed in the Spaces toolbar, if also another add-on is installed, which adds a button to this toolbar. In my case, it's Cardbook.

To Reproduce
Steps to reproduce the behavior:

1. Install and enable ThirdStats.
2. Install and enable Cardbook.
3. There are duplicate icons:
   

Expected behavior
There should be just one button for each add-on.

I reported the problem to Cardbook too. Their suggestion is to fix it on your side:
"it's because this add-on uses messenger.spacesToolbar.addButton which is now deprecated, it should use browser.spaces.create instead.."

System (please complete the following information):

• OS: win 11
• Thunderbird Version 128.4.1esr (64‑bit)