Apparently SG either didnt think about duplicate heroes or decided that they just would program the game not to give out family bonus to a duplicate hero…
They all get to eats, you have 1 Ariel, 1 Gadeirus, 1 Triton, and 2 Proteus which mean 4 unique heroes from Atlantis family. 4 unique = 15% defense bonus. All five heroes (1 Ariel, 1 Gadeirus, 1 Triton, and 2 Proteus) will receive the 15% defense bonus, no one is neglected here.