ok so i went into curseforge made a 1.16.5 minecraft world so then i got into adding modpacks 105 total now JEI happens to be one of em now when i press play for my world alls good it auto switches to the java launcher and i press play it pops up loads the mods but at the very end when its supposed to pop the main menu up i get this instead Fml.modloading.dupedmod and this was the crashreport
mc crash.txt

i just rlly dont know why i mean i mess with mods all the time everyday and dont get that Dupedmod crash thing and even im stumped g

Thanks for the report!
I think this just means you have multiple jei .jar files in your mods folder. Take a look and see if there's an extra one in there for some reason.