“Wouldn’t they reside ON Long Island, not “in” it? :
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Not necessarily-——Cuba is an island and people live “in” Cuba.
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It's an interesting distinction.
It would appear that one would live 'in' an island when it's a sovereign nation, but live 'on' an island when it's merely a part of a larger political union.
In England, Japan, the state of Hawaii, but -
On The Isle of Man, Okinawa, Oahu, etc.