Sudoku Players' Forum (clone)

Why the 2 sudokan's diagonals have always at less 2 equals elements between 1 to 9 each other ?
Who can proof this ?
Conjecture : Each sudokan's diagonal contains at less 2 same elements .

Actually, someone recently posted a variant (Magic Sudoku -- I can't find it now) which, when solved, followed the rules of a solved Sudoku grid and had a different digit for each place in both diagonals.