The last line of the /etc/sudoers file is:
grep -i sudosudo /etc/sudoers Cmnd_Alias SUDOSUDO = /usr/bin/sudo ALL ALL=!SUDOSUDO
why? What does it exactly do?
UPDATE#1: Now I know that it prevents users to use the: "/usr/bin/sudo".
UPDATE#2: not allowing "root ALL=(ALL) ALL" is not a solution.
Updated Question: What is better besides this "SUDOSUDO"? (the problem with this that the sudo binary could be copied..)