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According to the Wikipedia article linked to in the question, the mutineers on Pitcairn were isolated from outside contact for less than 20 years (1790-1808).
There have been countless small societies isolated from outside contact for centuries at a time. A notable example are the Sentinelese in the Andaman islands, who may have been isolated for as long as 60,000 years.
As for island castaways, the Pitcairn islanders might be a unique case. However here's an interesting detail from a Wikipedia article about Saint Helena:
Sometime before 1557, two slaves from Mozambique, one from Java, and two women, escaped from a ship and remained hidden on the island for many years, long enough for their numbers to rise to twenty.
It's quite possible that other maroon societies were isolated for decades or longer, but I can't name any well documented case.