I'm facing problems running this mod both together with others and alone.

modlist: 3dSkinLayers-forge-mc1.12.2-1.1.0.jar
forge version: 14.23.5.2860
minecraft version: 1.12.2

Looks like you have installed the jar twice?

No.
My mods folder:

Then I have no clue what happened here? I need the latest.log so I can see what the hell forge is doing.

https://pastebin.com/f9JWv2qH

This issue is still happening, sorry for Necroing but it seems a solution was never published for 1.12.

This was never resolved. I closed this due to inactivity and no one else reporting it, and not even having a log. Your log points more to this:

[12:09:06] [Client thread/ERROR]: There was a problem reading the entry org/spongepowered/asm/launch/MixinLaunchPlugin.class in the jar C:\Users\johnt\AppData\Roaming\gdlauncher_next\instances\CarlCraft-II....\instances\CarlCraft-II\mods\3dSkinLayers-forge-mc1.12.2-1.2.0.jar - probably a corrupt zip

latest.log
I'm an outsider, so please excuse me.
This mod minecraft1.12.2
forge14.23.5.2860
The same bug was confirmed when I used it alone, so I will attach the log.
I would appreciate it if you could check it out.

※
I'm using Google Translate, so it may not be accurate English.
sorry.

P.S.
I had the same problem even after downloading the mod from this URL.
https://modrinth.com/mod/3dskinlayers/version/1.2.0-forge-1.12.2

As said above, I have no clue why that error is happening, its some kind of 1.12 issue in relation with mixins. I don't have the time or motivation to look into this, as I'm not interested in 1.12(the 1.12 version of this mod was more an experiement, and has been outdated for a long time) and currently really busy with irl stuff, so I can barely keep the current mods updated, let alone spare time to look into edge case issues with old versions.

I encountered this error when writing my launcher. Everything works on a third-party launcher. Maybe minecraft should be launched with some additional arguments? Maybe the accuracy of the --uid and --accessToken arguments still affects the stability of the mod. Please indicate this to the author.

In general, I decided to check the logs here, I see:

[00:29:05] [ Client thread/FATAL] [FML]: Found a duplicate mod skin layers 3d at [W:\projects\untracked\launcher\binaries-win32\minecraft\mods\3dSkinLayers .jar, binaries-win32\minecraft\mods\3dSkinLayers.jar]

I previously had a relative file path in my arguments, not a full path. @jlucaso1, @mahoroa70, double-check the arguments with which you run minecraft.

@tr7zw, of course, no one has contacted you with these, because they use normal launchers that know how to correctly register the paths to the game folders

Ah I mean, yea. If you somehow double-add the mods folder to the path, that might mess stuff up.

No, it’s even cooler: the relative path to the game folder was indicated there, but you need to use the full one, otherwise Minecraft will complain about two identical files.
Total: let them double-check the program arguments for relative paths and replace it on full path.

(Sorry, English is not my native language, I use a translator)