arc patch-ing the same Differential twice gives you two different commits locally ... obviously with different commit hashes.
So if both John and Jill both pull a Differential into their respoctive local checkouts, they have no immediate way to see if they are looking at the same code, as they'll see different commit hashes.
(Note that they do see the number of the Differential (like D8), but they do not see the number of the Diff they received (E.g.: D8 has two different Diffs:
When trying to help people with git, typically my first question is “At which commit hash are you?”, so I can reproduce their environment. With that ability gone, helping others with git becomes shooting in the dark, as it's hard to reproduce the state of their checkout.
See also T162: Commit hashes of landed patches need not match the latest ones shown in Differential