a conjunctive normal form that is a tautology
Take any clause in the formula. If all variables in that clause are different, we can always assign $0$ to a non-negated variable and $1$ to a negated variable. This makes that clause $0$. Therefore, it makes the CNF formula $0$. Thus, tautology is not possible unless there is $A \vee \neg A$ in every clause.
This is probably what Kyle Jones stated in his comment.