rarity among rarities; rara avis; black swan
Emphatic set phrase meaning something extremely rare, literally 'rarity among rarities'. Used in both literal and figurative contexts.
This stamp is a rarity among rarities and hardly ever appears on the market.
An honest politician like him is a black swan.
A set phrase composed of 珍 (rare) + 中 (among) + の (possessive particle) + 珍 (rare), literally 'rarity among rarities'. The exact origin is uncertain, but it follows a common pattern of emphatic repetition in Japanese.