[Fate]

I think I read once (but have not confirmed myself) that any spell with over 100 fatigue will have the fatigue capped at 100 for the *Caster only*. So in communion, the slaves need to deal with the full fatigue of the spell, but the caster never need kill themselves casting a spell.

Which means in your case, the mage *should* be able to cast Magic Duel (50 fatigue) and then returning (to bring it up to 150 fatigue before they actually return).

In practice the AI might decide to use extra gems to reduce the fatigue from Magic Duel (spend another gem gives only 25) only to suddenly realize they don't have enough for returning (the AI does not think ahead when spending gems). So if you want a guarantee consider giving your mage 4 astral gems (or testing it beforehand).