Automatically overwrite a file if already exists with FFmpeg by using -y which means overwrite files without asking:

ffmpeg -i input.mp4 -c:v libx264 -crf 25 -preset fast -y output.mp4

The -y must be stated before defining the output.

Alternatively, you can use -n which means it won’t ask if you want to overwrite, instead it will exit.

ffmpeg -i input.mp4 -c:v libx264 -crf 25 -preset fast -n output.mp4