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-n (global)
Do not overwrite output files, and exit immediately if a specified output file already exists.

Usage:

$ ffmpeg -n -i input output.mp4
  File 'output.mp4' already exists. Exiting.
-n is a global option. Global options should be specified first.
The opposite option is -y which will automatically overwrite the output without asking.
Note: There is a bug that causes -n to not work in some cases, such as with the tee and image muxers. See bug report #8492: tee muxer silently overwrites output files. Until this bug is fixed it is recommended to test first or use your scripting language to check if the output exists.
                I ran ffmpeg -n -i input.vid output/%06d.png and it clobbered the pre-existing output files.
– Arnon Weinberg
                Oct 5, 2020 at 4:50
Ideally your script should check for the existence of the input and output files prior to you calling ffmpeg to do its (potentially) dangerous operation. By dangerous I mean overwrite existing files...
Once you have ascertained whether the output already exists your script can move on to the next file for processing.
                if you're using python, you can check if the file exists using pathlib  - stackoverflow.com/a/82852/1265980
– Ereli
                Jun 17, 2018 at 21:14