Use a for loop to iterate over the values in 1st column and if the required condition is satisfied, append the value in the 2nd column of respective row to the answer.
The following code does that.
data = [28 1; 28 2; 28,13; 30 11; 30 22; 30 30];
cur_val = data(1,1);
ans = [];
for i = 1:length(data)
if cur_val ~= data(i,1) % checking if it is the last repeating value in column 1
ans = [ans data(i-1,2)]; % appending the value in the 2nd column of the previous row to the ans
cur_val = data(i,1);
end
end
ans = [ans data(end,1)];
Hope it helps in solving your problem.
