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Spell Duplication duplicates all spells that have been cast in the current cast before it. If you have modifiers in front of Spell Duplication in the same cast state, Spell Duplication will act as a X multicast, where X is the amount of modifiers (Multicast spells works too, but the math becomes a bit more messy). Spell Duplication can't cast other instances of itself.

Useful guides:

Expert guide: Draw

The spell description can be rephrased as "Cast all spells in the Hand", which can't be understood without reading the guide above.