problem on concatenate files ffmpeg

[lomero]

i have 6 files with same resolution and same audio, 6 clips on same video. i want concatenate all clips into one output file.
i have used ffmpeg -f concat -safe 0 -i mylist.txt -c copy output.mp4
but have multiple error (this is only little part of my error):

[mp4 @ 0000000002967ec0] Non-monotonous DTS in output stream 0:0; previous: 17704187, current: 17696243; changing to 17704188. This may result in incorrect time stamps in the output file.
[mp4 @ 0000000002967ec0] Non-monotonous DTS in output stream 0:0; previous: 17704188, current: 17697243; changing to 17704189. This may result in incorrect time stamps in the output file.
[mp4 @ 0000000002967ec0] Non-monotonous DTS in output stream 0:0; previous: 17704189, current: 17698243; changing to 17704190. This may result in incorrect time stamps in the output file.
[mp4 @ 0000000002967ec0] Non-monotonous DTS in output stream 0:0; previous: 17704190, current: 17699243; changing to 17704191. This may result in incorrect time stamps in the output file.
[mp4 @ 0000000002967ec0] Non-monotonous DTS in output stream 0:0; previous: 17704191, current: 17700243; changing to 17704192. This may result in incorrect time stamps in the output file.
[mp4 @ 0000000002967ec0] Non-monotonous DTS in output stream 0:0; previous: 17704192, current: 17701243; changing to 17704193. This may result in incorrect time stamps in the output file.
[mp4 @ 0000000002967ec0] Non-monotonous DTS in output stream 0:0; previous: 17704193, current: 17702243; changing to 17704194. This may result in incorrect time stamps in the output file.
[mp4 @ 0000000002967ec0] Non-monotonous DTS in output stream 0:0; previous: 17704194, current: 17703243; changing to 17704195. This may result in incorrect time stamps in the output file.
frame=21415 fps=8683 q=-1.0 Lsize=  255203kB time=00:24:08.53 bitrate=1443.3kbits/s speed= 587x video:242436kB audio:12226kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.212552%

why this error? and how to solve? tanxs

PS i can play output video but have some break from one clip to another. little forward solve this break. but i want fluid play!