General :: How Does Du Determine Which Hard Link To Disregard

We have two directories:

$ ls -l
total 8
drwxr-x--- 2 nimmy nimmy 4096 Nov 15 19:42 jeter
drwxr-x--- 2 nimmy nimmy 4096 Nov 15 19:42 mariano

I create one file in the first folder:

$ dd if=/dev/zero of=jeter/zero_file.1 bs=512000 count=1
1+0 records in
1+0 records out
512000 bytes (512 kB) copied, 0.268523 s, 1.9 MB/s

This is the output of du:

$ du -sh *
504K jeter
4.0K mariano

As expected, if I place a hard link of the zero_file. in the other folder du output does not change:

$ ln jeter/zero_file.1 mariano/zero_file.2
$ du -sh *
504K jeter
4.0K mariano

there is nothing in the filesystem that points to zero_file.1 as the original file. So how does du know to count zero_file.1 but not zero_file.2?It cannot be a timestamp comparison because all hard links share one inode; they'll have the same timestamp data correct?