Copy & Convert Single Stream with FFmpeg

Assume you have a video with AC3 audio, but you want AC3 and AAC audio in the output.

The trick is the double -i to get another input to pump into -map 1:

$ ffmpeg -i video.mkv -i video.mkv \
  -map 0:1 -c:v:0 copy \                  # Copy video from stream 1
  -map 0:0 -c:a:0 copy \                  # Copy audio from stream 0
  -map 1:0 -c:a:1 libfdk_aac -ac:2 2 \    # Convert audio in stream 0 to AAC
  video.m4v