u263: repeated frames in the output avi file (at the beginning of the video)

[ami_stuff]

it looks like ffmpeg duplicates some frames at the beginning of the video in the output avi file
(does not happen with mov or mkv muxer)

​https://trac.ffmpeg.org/raw-attachment/ticket/1536/u263_b-frames_5.avi

C:\>ffmpeg -i u263_b-frames_5.avi out.avi

ffmpeg version 3.4.git Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 4.6.2 (GCC)
  configuration: --enable-gpl --disable-ffprobe
  libavutil      56.  0.100 / 56.  0.100
  libavcodec     58.  1.100 / 58.  1.100
  libavformat    58.  0.102 / 58.  0.102
  libavdevice    58.  0.100 / 58.  0.100
  libavfilter     7.  0.101 /  7.  0.101
  libswscale      5.  0.101 /  5.  0.101
  libswresample   3.  0.101 /  3.  0.101
  libpostproc    55.  0.100 / 55.  0.100
[h263 @ 0236d320] Warning: not compiled with thread support, using thread emulat
ion
Input #0, avi, from 'u263_b-frames_5.avi':
  Duration: 00:00:12.64, start: 0.000000, bitrate: 403 kb/s
    Stream #0:0: Video: h263 (U263 / 0x33363255), yuv420p, 320x240 [SAR 12:11 DA
R 16:11], 401 kb/s, 23.97 fps, 23.97 tbr, 23.97 tbn, 29.97 tbc
[h263 @ 024da020] Warning: not compiled with thread support, using thread emulat
ion
Stream mapping:
  Stream #0:0 -> #0:0 (h263 (native) -> mpeg4 (native))
Press [q] to stop, [?] for help
[mpeg4 @ 024db700] Warning: not compiled with thread support, using thread emula
tion
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf58.0.102
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 320x240 [SAR 12:11 D
AR 16:11], q=2-31, 200 kb/s, 23.97 fps, 23.97 tbn, 23.97 tbc
    Metadata:
      encoder         : Lavc58.1.100 mpeg4
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
[h263 @ 024da020] video_delay is larger in decoder than demuxer 1 > 0.
If you want to help, upload a sample of this file to ftp://upload.ffmpeg.org/inc
oming/ and contact the ffmpeg-devel mailing list. (ffmpeg-devel@ffmpeg.org)
    Last message repeated 513 times
frame=  259 fps=0.0 q=4.6 size=     262kB time=00:00:11.01 bitrate= 194.6kbits/s
[h263 @ 024da020] video_delay is larger in decoder than demuxer 1 > 0.
If you want to help, upload a sample of this file to ftp://upload.ffmpeg.org/inc
oming/ and contact the ffmpeg-devel mailing list. (ffmpeg-devel@ffmpeg.org)
    Last message repeated 79 times
frame=  298 fps=0.0 q=6.4 Lsize=     413kB time=00:00:12.64 bitrate= 267.9kbits/
s dup=0 drop=1 speed=22.5x
video:400kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing o
verhead: 3.216068%

[ami_stuff]

Replying to ami_stuff:

duplicates some frames at the beginning of the video

this is probably due to a bug in u263 encoder after all