ffmpeg - duration usage in input text file - ffmpeg

Question

I am trying to use ffmpeg to concatenate video segments with some black screen. To do that I've first generated a blank 10-second video (no audio track) with:
$ ffmpeg -f lavfi -i color=black:s=320x240:r=1 -f lavfi -i anullsrc -t 10 -vcodec libvpx -an blank.mkv
I then created the simplest possible scenario within input.txt file (contents below) in order to have three seconds of black screen followed by some video (no audio track):
file 'blank.mkv'
duration 3
file 'video_example.mkv'
And, finally, ran the following ffmpeg command to concat the contents of that input file:
$ ffmpeg -f concat -i input.txt -codec:v copy -codec:a copy output.mkv
The issue that I have is that the duration 3 is not considered, so the final video still has ten seconds of black frames (instead of three) followed by my video. And also "Non-monotonous DTS in output stream 0:0 ..." message is shown when using duration x in the file. If I remove duration the warnings are gone and getting the 10-second black screen first output as well.
Full output of the ffmpeg concat command:
$ ffmpeg -hide_banner -f concat -i input.txt -codec:v copy -codec:a copy output.mkv
Input #0, concat, from 'input.txt':
Duration: N/A, start: 0.000000, bitrate: N/A
Stream #0:0: Video: vp8, yuv420p(progressive), 320x240, SAR 1:1 DAR 4:3, 1 fps, 1 tbr, 1k tbn, 1k tbc
Metadata:
ENCODER : Lavc57.107.100 libvpx
DURATION : 00:00:10.000000000
File 'output.mkv' already exists. Overwrite ? [y/N] y
Output #0, matroska, to 'output.mkv':
Metadata:
encoder : Lavf57.83.100
Stream #0:0: Video: vp8 (VP80 / 0x30385056), yuv420p(progressive), 320x240 [SAR 1:1 DAR 4:3], q=2-31, 1 fps, 1 tbr, 1k tbn, 1k tbc
Metadata:
ENCODER : Lavc57.107.100 libvpx
DURATION : 00:00:10.000000000
Stream mapping:
Stream #0:0 -> #0:0 (copy)
Press [q] to stop, [?] for help
[concat # 000000000031a440] DTS 3000 < 9000 out of order
[matroska # 0000000000328420] Non-monotonous DTS in output stream 0:0; previous: 9000, current: 3000; changing to 9000. This may result in incorrect timestamps in the output file.
[matroska # 0000000000328420] Non-monotonous DTS in output stream 0:0; previous: 9000, current: 4001; changing to 9000. This may result in incorrect timestamps in the output file.
[matroska # 0000000000328420] Non-monotonous DTS in output stream 0:0; previous: 9000, current: 4998; changing to 9000. This may result in incorrect timestamps in the output file.
[matroska # 0000000000328420] Non-monotonous DTS in output stream 0:0; previous: 9000, current: 6004; changing to 9000. This may result in incorrect timestamps in the output file.
[matroska # 0000000000328420] Non-monotonous DTS in output stream 0:0; previous: 9000, current: 7002; changing to 9000. This may result in incorrect timestamps in the output file.
[matroska # 0000000000328420] Non-monotonous DTS in output stream 0:0; previous: 9000, current: 8005; changing to 9000. This may result in incorrect timestamps in the output file.
frame= 5794 fps=0.0 q=-1.0 Lsize= 7109kB time=01:37:09.70 bitrate= 10.0kbits/s speed=5.16e+004x
video:7043kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.926229%
Any idea what am I doing wrong? The warning seems to hint towards the issue here.
Other possibly useful info:
$ ffprobe -hide_banner blank.mkv
Input #0, matroska,webm, from 'blank.mkv':
Metadata:
ENCODER : Lavf57.83.100
Duration: 00:00:10.00, start: 0.000000, bitrate: 1 kb/s
Stream #0:0: Video: vp8, yuv420p(progressive), 320x240, SAR 1:1 DAR 4:3, 1 fps, 1 tbr, 1k tbn, 1k tbc (default)
Metadata:
ENCODER : Lavc57.107.100 libvpx
DURATION : 00:00:10.000000000
$ ffprobe -hide_banner video_example.mkv
Input #0, matroska,webm, from 'video_example.mkv':
Metadata:
encoder : GStreamer matroskamux version 1.8.1.1
creation_time : 2018-05-04T17:57:04.000000Z
Duration: 01:37:08.70, start: 15434.269000, bitrate: 9 kb/s
Stream #0:0(eng): Video: vp8, yuv420p(progressive), 320x240, SAR 1:1 DAR 4:3, 1 fps, 1 tbr, 1k tbn, 1k tbc (default)
Metadata:
title : Video
$ ffmpeg -v
ffmpeg version 3.4.2 Copyright (c) 2000-2018 the FFmpeg developers
built with gcc 7.3.0 (GCC)

Answer 1

For video and audio files, inpoint/outpoint have to be used.
file 'blank.mkv'
outpoint 3
file 'video_example.mkv'
duration is useful for single images, like when making a slideshow, or raw audio/video streams where ffmpeg can't reliably discover the media duration.