It is one of the property of determinants. If, we have any matrix with two identical rows or columns then its determinant is equal to zero. We can verify this property by taking an example of matrix A such that its two rows or columns are identical.
After taking example of such a matrix, we find its determinant using co-factors and its determinant comes out to be zero at the end.
To understand this property with example, go through the file present below. I have provided example of such a matrix and have calculated it determinant which comes out to be zero.