"Official 100%" category duping summary:

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Tried all possible variations, crashes the game. https://pastebin.com/6mDsLp6n

There are two ways:
1) Make sure two cs play at the same time at Catalina's place
(two "Gone Courting" cs) and then skip. One mission gets stuck - that mission
be turned into a seconds - say Local Liquor Store (like BP duping by starting it
during Quarry).
That gives you 4*3 "combinations", (e.g. start LLS during stuck Gone Courting).
E: I am pretty sure that it does not matter which intro cs you use, so there are only
3 combinations (4 if you count Tanker Commander).
2) Make sure no cs gets stuck. This only gives you 4 combinations. It simply does
not matter whether you use Gone Courting or First Base or whatever.

Note that you can """"fake-dupe"""" - get om0 during a "main mission".
For that, you dupe the "first mission"
(like Gone Courting or First Base) outside the cs and then you start the "main mission"
(like Local Liquor Store) and make sure to leave the "marker" right after (before
the fade ends). You get onmission 0 and one Local Liquor Store, but cannot start
another instance by goind into a heist trigger (those marked with a dollar sign).

works
If you Trucking start it, one instance "restarts" Snail Trail after ending
(passing/failing) it.
It would be nice if you could save backtracking that way but I do not know of a way
to only complete the "restarting" instance but not the other instance as well. And two
or more Snail Trail passes lock you out of completing the game.

Explanation: It returns to the same offset that Trucking would normally return to
if you finished it (instead of turning Trucking into Snail Trail). But for Snail Trail,
at that offset it starts the main part of the mission.

Snail Trail seems almost unique - you could restart a mission starting from the middle of the
"thread" command but that (almost??) always crashes.

two gates, no way you can jump in there
else, it would work fine (thanks Reset)

You can dupe it Los Desperados style and take the call during the second instance.
For that you need the "Valet trick" which is slow to set up.

Whole thread can be skipped w/ duping Toreno's Last Flight + 0* Glitch

works

There are two routes:
- in one, you dupe BP right after skipping the first cs (when you just start it again in the cutscene, one instance gets stuck waiting for
the cutscene to end)
- in the other one, you convert a Quarry thread into a Black Project thread by
starting BP while Quarry runs

I also tested it with two Quarry instances once because if you dupe Explosive Situation you
get two Quarry starter threads and therefore get two Quarry instances. It also works fine.

Up to 12 instances are confirmed to work.
With too many instances the game eventually crashes
from too many soldiers spawned etc.

when duped outside the cutscene, it crashes at the start
if you dupe it inside the marker via teleporter (no other way available atm) it crashes
at the end
both have to do with a container that is rotated for the entire mission.
I forgot the reason it crashes if you dupe it outside the cutscene but it seems
unavoidable.
If you dupe it inside the cutscene, you can pass one instance, but that
calls mission_cleanup which de-spawns the container. The other instance then
tries to rotate a non-existent container, causing the crash