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It's rare to run into a celebrity by chance.
Literal
Famous-people [with-と] coincidentally meet-[nominal-のは] rare.
偶然 ('coincidence, by chance') functions adverbially here. 珍しい ('rare, unusual, uncommon') is an i-adjective used as a predicate for the nominalized 会うのは. The whole structure is [verb]-のは [adjective]: 'X-ing is rare/interesting/common.'