I've had the same question and this is how my chemistry teacher answered: 3d is a sub energy level of the 3rd energy level and 4s is a sub energy level of the 4th energy level. When electrons fill into the Fe atom they fill according to the Auffbau principle, in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d. But when 2 electrons are to be removed to form the Fe2+ ion, the electrons are not removed from the 3d orbitals. 3d belongs to the 3rd energy level which becomes an inner filled energy level when electrons are present in the 4th energy level. So electrons are removed from the 4s sub energy level, without disturbing the fully filled 3rd energy level.
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