Square Matrix with Duplicate Rows has Zero Determinant

Theorem
If two rows (or columns) of a square matrix are the same, then its determinant is zero.

Proof
From Determinant with Rows Transposed, if you swap over two rows of a matrix, the sign of its determinant changes.

If you swap over two identical rows of a matrix, then the sign of its determinant changes from $$D$$, say, to $$-D$$.

But the matrix is the same.

So $$D = -D$$ and so $$D = 0$$.