Sadly, it's not working for me.
Pressing [Alt]+[Shift]+D just opens the menu bar as if I was just pressing [Alt].

(using Firefox 99.0.1 on Windows 10)

I think it has something to do with localization. I use a german translation, where the "File" menu is called "Datei". So [Alt]+D opens that Dropdown for me ignoring [Shift].

It says in the description, that "Shortcut is configurable.", but I cannot find where.
//Update: I found it, thanks to the Screenshot. I'm just stupid. Sorry.

I changed it to [Alt]+[Shift]+K in manifest-json and created the extension myself for self-distribution.

Works great. Use it all the time.

Does what it says. Plain and simple.

Good plugin... thank you very much and have a nice day!

I'm sure I had this working on another laptop, but I have an issue on this, the left alt-key somehow doesn't work for user applications (window manager, etc. no problem, haven't fixed it yet), but no problem, I tried to reconfigure it:
Ctrl+Shift+D - Well, that just opened "add bookmark" popup - that was warned against though, not being able to override browser bindings.
Then I tried pretty much any Ctrl+Shift+[key] - none worked.
Btw, I tried Ctrl+Shift+. and worder if with shift key in the mix I should try ':' and not '.'.

So the issue is, this is useless. I have another add-on, Saka Key, which is really for full keyboard browsing, but the tab tublicate on it doesn't work the way I want with Tree Style Tab. Maybe yours would...?

As of it now, it just seems it only works with Alt, not with Ctrl. Either that or something in my FireFox is messing with it. At the moment it is useless on my main PC :(

Thank you for duplicate in background!

Duplicates a tab. Thats it.

Works like a charm

Just got it, changed shortcut, tested it, it works. Yay!

1/10/2020 Update
It stopped working a few months ago. Don't know if there was a FF update. All it does now is open a new tab, no duplication. Using FF 72.0.1 in LinuxMint.

I'd prefer a configurable shortcut key but this extension does what it says it will do.

Good, but the duplicated tab doesn't include the tab history. Can we make the new duplicated tab exactly the same (with all history etc) as the original tab? This is required to enable us to navigate back in the duplicated tab if we need to.

Not working with firefox 59

Very unreliable - works once then fails in Firefox

Duplicate tab is an essential feature Mozilla!