> 1a) If you have a harmony remote, lower the repeat value in the harmony software: in the remote, go to the troubleshooter and lower the value to 0 or 1
The value was set to 1, I changed it to 0. If anything, this made the problem worse.
> 2a) raise the interkey_delay in the flirc advanced software
The value was already set to 6, which is the maximum.
To be honest, I doubt that these settings are targeting the root cause of the problem. It would make sense that a higher delay / lower repeat value helps with double key presses, but the problem here is that one physical key press triggers infinite key presses in the software.
Maybe it's not even a bug in your firmware but in the RPi service that handles the messages that are sent via flirc? But then I don't understand why a Logitech K400 is working flawlessly.