Why does helium seem to exhibit an ionization energy affected by the shielding of electrons?
The premise is problematic as there is no 'shielding' effects in an isolated Helium atom.

However, Phillipp was correct in explaining why the ionization energy of Helium is not exactly twice that of Hydrogen.  This is because the energies of each electron are higher due to their correlated nature (i.e. they repel each other into higher, non-ground-state-like energies).  Therefore, it takes LESS energy to rip one of those electrons out of the system.