flac to mp3 with ffmpeg leads to warning "Frame rate very high for a muxer not efficiently supporting it."

FFmpeg will treat attached pictures such as cover images as regular streams and its MP3 muxer defaults to constant frame-rate which is leading to frame duplication. The solution is to change video sync method to passthrough, so

for file in *.flac; do \
   ffmpeg -i $file \
   -vsync 0 -codec:a libmp3lame -qscale:a 6 \
   "${file%.*}.mp3"; \
done