No need to run it twice. Basic syntax is ffmpeg -i video -filter_complex "[0]split[vid][pal];[pal]palettegen[pal];[vid][pal]paletteuse" out.gif ffmpeg 4.

No need to run it twice. Basic syntax is ffmpeg -i video -filter_complex "[0]split[vid][pal];[pal]palettegen[pal];[vid][pal]paletteuse" out.gif ffmpeg 4.0 and later will automatically insert fifo buffers for the main video while one copy is analyzed to generate the palette.