[m,n]=size(D);
    for i=1:m-1
    if D(i,1)==D(i+1,1)
    D=reshape(D, 2, m/2*n);
    D=(D(i,:)+D(i+1,:))/2;
    D=reshape(D, m/2, n);
     else
      end
    end

for row=0,row++

    if ( m[row,1]  == m[row+1,1])
    {
        mean1 = mean(m[row]);
        mean2 = mean(m[row+1]);
        mean = mean(mean1,mean2);
    }

end for

index_row = 1:1:size(D, 1); % linear row index
index_valid = all(~diff(D(:,1:2)), 2); % valid rows (logical)

mean_matrix = (D(1:end-1,:) + D(2:end,:))/2; % matrix of all means

% matrix of valid mean rows only
mean_matrix_valid = mean_matrix(index_valid,:); % logical index
% linear index of valid rows, i.e. the pairs indexed (i, i+1)
index_row_valid = index_row(index_valid); % valid rows (int)

D = [1 2 3 4 5; 1 1 1 1 1; 1 2 4 4 4; 1 2 3 3 3; 2 2 2 2 2; 2 2 3 3 3];

>> D =
     1     2     3     4     5
     1     1     1     1     1
     1     2     4     4     4
     1     2     3     3     3
     2     2     2     2     2
     2     2     3     3     3

>> index_valid =

     0
     0
     1
     0
     1

>> index_row_valid =

 3     5

>> mean_matrix_valid =

    1.0000    2.0000    3.5000    3.5000    3.5000
    2.0000    2.0000    2.5000    2.5000    2.5000