Parameter-free theoretical predictions based on a dual shell mechanism within the proxy-SU(3) symmetry of atomic nuclei, as well covariant density functional theory calculations using DDME2 indicate that shape coexistence (SC) particle-hole excitation cannot occur everywhere nuclear chart but is restricted islands lying regions 7–8, 17–20, 34–40, 59–70, 96–112, 146–168 protons or neutrons. Syst...