ractrot's discovery, afaik.

Pretty fast. Basically you start the first one normally, and for the next four you jump (whilst holding W or S, depending on which way you're facing) and then after some time press F3 quickly thrice and it should work. [done with a 2:27 waiting period after saving the game in the video, though 2:25 also works. 2:26 is the ideal middle-ground between going fast and being safe, imo.]

I find that jumping only when the text (mission instruction) appears is a good way of ensuring it working; while going a few frames earlier may technically work because of the time it takes to jump, it's pretty YOLO and not really worth it. Also, be sure to be pressing F3 quickly in succession to give a higher chance of it working, though that might require SSD or RAM Disk.

Another tip for getting this consistently is to make the jumps a bit on the edge of the marker rather than closer to the middle. This gives you more leeway for pressing F3 and this makes this a HELL of a lot more consistent.

I tried to do like on video about a month ago, but I found it difficult. Maybe I just should practice it.

Well, I to practice a little, for now it's still hard for me, but this duping is 100% faster. Thank you for showing me that strats is possible!

Did some macro testing and this only seems to be consistant with a delay of 5-7 frames in between jumping and pressing F3 for the first time. Also all F3 presses have to be at least within 2 frames of each other. But I don't know how much the length of the replay affects this theory.