Hi, this addon has never provided CTRL + K as a shortcut. I believe that shortcut already is used by Firefox to select the address bar, it does on mine. This addon provides CTRL+SHIFT+D which I've verified works on several linux systems and the CMD equivalent on Mac. I've also verified the ALT+SHIFT+D shortcut working on Windows. I didn't choose 3 key shortcuts deliberately. The reason they are both 3 keys is to avoid clashing with existing shortcuts in Firefox as much as possible. I wanted to allow a customisable shortcut and this remains a goal. It simply is not possible in the current WebExtensions options. The keys of the shortcuts have to be specified in the configuration of the addon, and can only be detected somewhat dynamically. Once I am able to register keyboard shortcuts from the settings page I plan to implement them.

If you need an existing solution now you can look into AutoHotKey on Windows or AutoKey on Linux (there are probably similar programs on Mac) and write a simple script to listen to a custom keyboard shortcut like your CTRL + K. An external program should then be able to detect keypresses that Duplicate Tab cannot (due to Firefox clashing with them) and simulate pressing one of the existing keyboard shortcuts of this addon to trigger the desired behaviour.

Alternatively you can edit the manifest.json of Duplicate Tab's source code to use a different shortcut and use the modified addon instead.

I am open to adding extra keyboard shortcuts, but they need to clash with Firefox as little as possible. I was never able to make CTRL+D work for example, which would have been the obvious choice.

Edit:
Firefox 60 looks like it will have customizable keyboard shortcuts available so hopefully we don't have to wait much longer after all: https://github.com/Skeletonxf/duplicate-tab/issues/2

Edit 2:
From Firefox 60 you should now be able to customize the shortcuts in browser with this addon, I still doubt CTRL+K will work however.