5715: Largest Submatrix 3

You are given matrix a of size n×m, its elements are integers. We will assume that the rows of the matrix are numbered from top to bottom from 1 to n, the columns are numbered from left to right from 1 to m. We will denote the element on the intersecting of the i-th row and the j-th column as a_ij.
We'll call submatrix i₁,j₁,i₂,j₂ (1≤i₁≤i₂≤n;1≤j₁≤j₂≤m) such elements a_ij of the given matrix that i₁≤i≤i₂ AND j₁≤j≤j₂. We'll call the area of the submatrix number (i₂-i₁+1)·(j₂-j₁+1). We'll call a submatrix inhomogeneous, if all its elements are distinct.
Find the largest (in area) inhomogenous submatrix of the given matrix.

3 3
1 3 1
4 5 6
2 6 1

6

3 4
5 2 3 1
3 3 5 3
4 4 4 5

4

2 6
1 2 3 4 5 6
8 6 7 8 9 1

8