How to convert MONO audio to STEREO audio from a Gnome SimpleScreenRecorder video

It's very easy:

Let me explain how to use it:

• input.mp4 is the file to be converted. In my case, because it was generated by SimpleScreenRecorder, in fact it doesn't contain a mono audio track, but a STEREO track but where only one of the two channels has audio, the other channel is empty or in "silence". So, technically is stereo and it has 2 audio channels: c0 and c1
• output.mp4 is the file that you want to get in real stereo, with both channels containing the same audio copied from the original non-empty channel
• in my case, the channel c1 was the non-empty, so in the parameter -af i put this: "c0=c1" to put an exact copy of the content at c1 in the c0 channel, and also "c1=c1" because i want to keep the channel c1 just as in the original video file.

I love ffmpeg... its beatiful never end. 🤩