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It’s quite simple actually, all you need is ImageMagick and ffmpeg.
You first extract all frames from the flv/video using ffmpeg, first you make a directory named whatever
ffmpeg -i video.flv -vcodec png -an -af rawvideo -r 8 whatever/%0d.png
Then you copy over the frames of the part to a separate folder, e.g. 1, then
convert -set disposal Previous -delay 10 -loop -1 1/*.png 1.gif
All in the command prompt/terminal