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 $D = 0$.