title: FFMPEG Concat protocol Error: Found duplicated MOOV Atom. Skipped it
body: <p>I'm concatenating mp3 files into a single m4a. </p>
  
  <p>To save time, I'm doing the mp3 to m4a conversion first (multi-threaded) and then concatenating the output files (m4a) into a single m4a file. </p>
  
  <p>The concat works when I do the conversion (mp3 -> m4a) and cancat in one swoop. But it takes forever to run. </p>
  
  <p>Example:</p>
  
  <pre><code>ffmpeg -i "concat:001.mp3|002.mp3|003.mp3" -codec:a aac -c copy output.m4a
  </code></pre>
  
  <p><br /><br />
  If I take those same mp3 files and convert them to m4a in parallel and then concat, I can save myself a significant amount time.</p>
  
  <p>The problem is if I run the concat command on the converted m4a files</p>
  
  <p>Example cmd :</p>
  
  <pre><code>ffmpeg -i "concat:001.m4a|002.m4a|003.m4a" -c copy output.m4a
  </code></pre>
  
  <p>I getting the following error:</p>
  
  <blockquote>
    <p>[mov,mp4,m4a,3gp,3g2,mj2 @ 000000000052f680] Found duplicated MOOV
    Atom. Skipped  it</p>
  </blockquote>
  
  <p><br /><br/>
  <strong>UPDATE</strong>: Ouput when concatenating m4a files (two track example):</p>
  
  <pre><code>C:\_ffmpeg&gt;ffmpeg -i "concat:001.m4a|002.m4a" -c copy output.m4a
  ffmpeg version N-79630-g9ac154d Copyright (c) 2000-2016 the FFmpeg developers
    built with gcc 5.3.0 (GCC)
    configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
  isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
  le-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --
  enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-lib
  modplug --enable-libmfx --enable-libmp3lame --enable-libopencore-amrnb --enable-
  libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enabl
  e-libschroedinger --enable-libsnappy --enable-libsoxr --enable-libspeex --enable
  -libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --ena
  ble-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx
  264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable
  -lzma --enable-decklink --enable-zlib
    libavutil      55. 22.101 / 55. 22.101
    libavcodec     57. 38.100 / 57. 38.100
    libavformat    57. 34.103 / 57. 34.103
    libavdevice    57.  0.101 / 57.  0.101
    libavfilter     6. 44.100 /  6. 44.100
    libswscale      4.  1.100 /  4.  1.100
    libswresample   2.  0.101 /  2.  0.101
    libpostproc    54.  0.100 / 54.  0.100
  [mov,mp4,m4a,3gp,3g2,mj2 @ 000000000048a9a0] Found duplicated MOOV Atom. Skipped
   it
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'concat:001.m4a|002.m4a':
    Metadata:
      track           : 1
      major_brand     : M4A
      minor_version   : 512
      title           : 001 Moon Over Soho
      artist          : Ben Aaronovitch
      album_artist    : Narrated by Kobna Holdbrook-Smith
      composer        : Narrated by Kobna Holdbrook-Smith
      album           : Moon Over Soho
      date            : 2012
      encoder         : Lavf57.34.103
      genre           : Speech
      copyright       : Tantor Audio
      compatible_brands: isomiso2
    Duration: 00:44:03.83, start: 0.000000, bitrate: 251 kb/s
      Stream #0:0(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, flt
  p, 128 kb/s (default)
      Metadata:
        handler_name    : SoundHandler
  [ipod @ 000000000048bb20] Using AVStream.codec to pass codec parameters to muxer
  s is deprecated, use AVStream.codecpar instead.
  Output #0, ipod, to 'output.m4a':
    Metadata:
      track           : 1
      major_brand     : M4A
      minor_version   : 512
      title           : 001 Moon Over Soho
      artist          : Ben Aaronovitch
      album_artist    : Narrated by Kobna Holdbrook-Smith
      composer        : Narrated by Kobna Holdbrook-Smith
      album           : Moon Over Soho
      date            : 2012
      compatible_brands: isomiso2
      genre           : Speech
      copyright       : Tantor Audio
      encoder         : Lavf57.34.103
      Stream #0:0(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, 128
   kb/s (default)
      Metadata:
        handler_name    : SoundHandler
  Stream mapping:
    Stream #0:0 -&gt; #0:0 (copy)
  Press [q] to stop, [?] for help
  size=   41892kB time=00:44:03.80 bitrate= 129.8kbits/s speed=2.21e+003x
  video:0kB audio:41445kB subtitle:0kB other streams:0kB global headers:0kB muxing
   overhead: 1.077122%
  
  C:\_ffmpeg&gt;pause
  Press any key to continue . . .
  </code></pre>