GSF abjuration shield + mage armor
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An easier solution to this would be to make this only apply if the caster is a specialist abjurer. That prevents any issues around losing hour/level mage armor when casting shield otherwise.
The spell's intended to give specialist abjurers access to mage armor spell since they lose it via specializing (which makes no sense), this solves the issue and complaints some people are having about the lowered duration of mage armor via it overwriting the shield spell.