There is some weird thing I don't understand:
I have 2G block.img file with an ext4 filesystem inside. There is a 2G test.txt file in block.img. test.txt consists of letters 'a'. Then I mount block.img to /mnt/first. And I can see this test.txt with letters 'a' inside. All is fine.
Weird thing begins here: simultaneously I mount block.img on /mnt/second and change all 'a' to 'b' in this text file. And I can see two different 2G files. I have two files 2G (4G used space) on one 2G partition! Unbelievable (:
Therefore, there is some mistake in my words, unfortunately I can't find it.