I noticed when I was trying to launch mongod from the /etc/mongod.conf file it gave me the error that security.authorization was duplicated and thus would not launch.
To fix this, I launched the conf file using
sudo vi /etc/mongod.conf and removed the duplicated setting. If you’re not a linux user (like myself) then to save and close the file, press Esc (may not be necessary) and type in
:wq. Or to simply close without saving, type