[Nader Douik]

how would an expression even be able to tell in which comp it resides, since it is actually in 2 at the same time?

It is not in 2 at the same time.

Method 1: There is only one copy of the precomp and the expression will check where it resides from the list of comps you give it to check against.
Method 2: Making a duplicate, like you said, that references the same composition just twice. That way you don’t need to check where the precomp resides. But with this method you can’t change the checkboxes from the duplicate on the main comp, but rather from the main copy in the P-C precomp. Being able to control the checkboxes from the main comp would be more convenient, for that method 1 is better.

[Nader Douik]

Thanks for taking the time Robert!
You’re right I forgot to clarify that! “Picture1” is a comp and not a layer.

I am not working with essential graphics or master properties, I just duplicate the comps once and then use them as they are each time I start working on a new video.

I don’t recognize the authority of luck.
I have made a system that works already, an expression that checks if my comp “picture1” is located in P-C or in the MAIN comp, and update the other expression so that it references the comp correctly. It works. But out of curiosity, I will keep looking if there is another way to do this.
Also found another very simple way to do this which is to just leave the comp “picture1” in P-C, and make a copy of it in the MAIN comp. That way, you will get the contents of “picture1” in the main comp, and you can control the expressions, checkboxes or whatever you have on “picture1” from the copy that exist in the P-C comp without needing to reference it again.