Block normalization of histogram of oriented gradient
I stucked at this question too, because overlapped blocks will be divided twice, right?

and so I found these :

1 http://www.geocities.ws/talh_davidc/#cst_extract

2 https://stackoverflow.com/questions/32417531/hog-what-is-done-in-the-contrast-normalization-step

to verify my answer, I done this: 
on 1, they using image size HOG126x63(), block size is 18x18, with 50% overlapping blocks means it had 9px stride. 
126/9, 63/9 = (13, 6) blocks.

they mentioned that final vector would be 6318 = 78*81 == 78*9*9 (78*number_of_cells_in_one_block*number_of_bins) 
turns out, 13*6 = 78, so final vector is 
(blocks_in_image_col *blocks_in_image_row *number_of_cells_in_one_block *number_of_bins)

conclusion is, one block is counted as one set vectors (2*2*9 as in image) , moving the blocks will produce one set of vectors again. there won't be cells normalized twice (it will make the histogram spike at first and flat on remaining due to recursive dividings)