Probably restart your editor when that happens. If it still doesn't work, go to Editor -> Editor Settings, and to the Shortcuts tab. You can search for "Duplicate" and "Below" to find the corresponding commands and assign a shortcut to them.
OK it is M in my Godot
Seems it's not set to the default value. I've also had some issues in some versions with the shortcut not working. Shortcuts with Alt don't work well on Linux, like the one to fold code.
can i the copied tiles flipping before i paste it?
Hi Micha. I'm unsure I got your question, but if it is "if I have a selection with flipped tiles, can I copy them in that state?"
The answer's yes. When you paste your copied tiles, they'll be the same as what you copied.
If the question is "can I flip my selection", the answer is no, I just tested this and it doesn't seem possible at the moment.