Quick Ionisation Energy Question

The Ne atom and the Mg 2+ ion have the same number of electrons. Give two reasons why the first ionisation energy of neon is lower than the third ionisation energy of magnesium.

I can think of the obvious first reason which is that for the third IE of magnesium, you are removing an electron from a positive 2+ ion, which requires more energy than removing an electron from a neutral atom as is the case with the first IE of neon. But what is the second reason - isn't the distance between the outermost electron and the nucleus, as well as shielding, now the same?

just go for the number of protons holding onto the electrons

Isn't that the same as the first reason?