Is this swordfighting Sada a male or a female? (Sabre used to be men-only, but now it's both, in these Olympics.) If it's male, I've got no idea. But if it's female, then my guess would be that Sarah has two nickname forms: Sally and Sadie. And I can readily imagine parents, possibly fond of Sarah but thinking it too popular, playing round with the nicknames and getting Sada.
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