The greeting "Moin" is of Frisian origin. Moin originates from the greeting "Moije Daag", which just means "good day". In this way Moin is used today in northern Germany, especially in and around Hamburg, and interestingly has spread over some IRC channels as the standard greeting.
"Moin Moin" is supposed to be an emphasized Moin. Some people regard this as overdoing, though.