karma and moral justice

[nameless]

Doesn't Karma provide the same effect for two identical causes?

not necessarily, the process actually takes place inside the mind. If doer performs it without being mentally violent, and none of them bothers about it, there would be no effect.

If two individuals commit the same sin, does Karma say they may experience completely different consequences?

the consequences may appear different, but it would provide the same suffering.
There are people who become depressed for some silly reason.