Nincada evolves into both Ninjask and Shedinja under one condition.
If you want a Shedinja, have at least one PokeBall, and one extra spot in your party when Nincada evolves at Lv. 20. If you do have this, you will get the Ninjask that Nincada evolves into, and that extra spot will have a Shedinja.
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