Alneyan listed this as an issue, which could be a bug or just an undocumented feature.
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				- It appears that Magic Duel uses *base* astral level: spells and items do not seem to affect the spell at all, or they might be limited in power: an Astral 8 mage (with +1 coming from a spell) lived through more than 500 Magic Duels, while an Astral 8 mage, with +2 coming from spells, was killed after the 10th Magic Duel or so. An Astral 3 mage, fully loaded with items (reaching Astral 8), was killed after the third Magic Duel. Thanks to JK for the information. 
			
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 Johan K. gave this answer:
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				* The thing with magic duel is that it uses your raw magic level. Items and enchantments have no effect. So the spell is probably working fine. 
			
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 I remembered every booster wasn't included, but when I found the source, I was quite shocked. 
I really think the spell description should be changed to reflect this.