title: ffmpeg concat failing with dts timing error
body: <p>I have some videos that I am trying to concat with ffmpeg, they all work just fine by themselves but trying to concat them with this command:</p>
  <pre><code>ffmpeg -f concat -safe 0 -fflags flush_packets -fflags discardcorrupt -i ./videos.txt -c copy -an 1.mp4
  </code></pre>
  <p>This isn't a live stream or mpeg-dash, it's just standalone mp4's that I would like to join into one longer video.</p>
  <p>It's just a constant stream of errors like this:</p>
  <pre><code>[mp4 @ 0x5560664a1780] Non-monotonous DTS in output stream 0:0; previous: 72811593, current: 4396274; changing to 72811594. This may result in incorrect timestamps in the output file.
  [mp4 @ 0x5560664a1780] Non-monotonous DTS in output stream 0:0; previous: 72811594, current: 4396786; changing to 72811595. This may result in incorrect timestamps in the output file.
  [mp4 @ 0x5560664a1780] Non-monotonous DTS in output stream 0:0; previous: 72811595, current: 4397298; changing to 72811596. This may result in incorrect timestamps in the output file.
  [mp4 @ 0x5560664a1780] Non-monotonous DTS in output stream 0:0; previous: 72811596, current: 4397810; changing to 72811597. This may result in incorrect timestamps in the output file.
  
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x55e9b217ab40] Dropped corrupted packet (stream = 0)
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x55e9b217ab40] stream 1, offset 0x402a6: partial file
  ./videos.txt: Invalid data found when processing input
  </code></pre>
  <p>Just to be clear these aren't .ts videos but mp4 videos.</p>
  <pre><code>Here's an example of ffprobe from
        Metadata:
          minor_version   : 512
          major_brand     : isom
          compatible_brands: isomiso2avc1mp41
          comment         : vid:v09044820000brpbed8biap950ufpcr0
          encoder         : Lavf58.20.100
        Duration: 00:00:14.70, start: 0.000000, bitrate: 901 kb/s
          Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 576x1024 [SAR 1:1 DAR 9:16], 830 kb/s, 29.97 fps, 29.97 tbr, 11988 tbn, 59.94 tbc (default)
          Metadata:
            handler_name    : VideoHandler
          Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 64 kb/s (default)
          Metadata:
            handler_name    : SoundHandler
  </code></pre>
  <p>another of the video file</p>
  <pre><code>  Metadata:
      minor_version   : 512
      major_brand     : isom
      compatible_brands: isomiso2avc1mp41
      comment         : vid:v090447e0000bo8lplu9uukrbn9qbupg
      encoder         : Lavf58.20.100
    Duration: 00:00:12.12, start: 0.000000, bitrate: 791 kb/s
      Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, smpte170m/bt470bg/smpte170m), 576x1024, 718 kb/s, 30 fps, 30 tbr, 1000k tbn, 60 tbc (default)
      Metadata:
        handler_name    : VideoHandler
      Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 64 kb/s (default)
      Metadata:
        handler_name    : SoundHandler
  </code></pre>