Why does A/B testing cause 'dedupe by email' to fail when sending CiviMail to contacts with same email address
From what you describe it sounds like the deduping is happening within each group rather than before the recipients are split into A, B and final groups.

You could confirm this by doing:

Test 4: 8 contacts sharing same email with A/B - 25% to A and 25% to B and 50% to FINAL

I would expect the same results as for test 3: 1 delivered to A mailing, 1 delivered to B mailing, and 1 delivered to Final mailing.