title: ffmpeg concat produces DTS out of order errors
body: <p>I'm following the documentation on how to <a href="https://trac.ffmpeg.org/wiki/Concatenate" rel="noreferrer">concatenate</a> files with ffmpeg but during the process I'm seeing lots of warning and the output video stops after the first chunk but the audio keeps on playing.</p>
  <p>This is the command I'm using to concatenate the files:</p>
  <pre><code>ffmpeg -f concat -i mylist.txt -c copy output.webm
  </code></pre>
  <p>This are the warnings I'm seeing:</p>
  <pre><code>[concat @ 0x7fee11822a00] DTS 0 &lt; 2500 out of order
  [webm @ 0x7fee11011000] Non-monotonous DTS in output stream 0:0; previous: 2500, current: 0; changing to 2500. This may result in incorrect timestamps in the output file.
  </code></pre>
  <p>The video files are coming from an external source. My current solution is to re-encode every file separately to mp4 and then to concatenate them together and then to re-encode the complete file back to webm. That, of cause, that takes some significant time but I can't find another solution.</p>