Why did the image duplicated 3 times?

Shu - it looks like your original image is in colour so consider this line of code

[x,y] = size(ori_roi{i});

i suspect that y is three times what it should be...and so your image is replicated three times. For example,

Z = randi(255,100,50,3);
[x y] = size(Z);

Where x is 100 (correct) but y is 150 (incorrect!). And so the code

for a=1:1:x
    for b=1:1:y
        C(a,b) = Z(a,b) + 0;
    end
end

[x y, ~] = size(ori_roi{i});

so that the third dimension is ignored and your x and y should now be the correct height and width of your original image.