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Duffie and Nanai have committed to the Blues through to the end of 2019, while Collins has extended his contract for another season.
Collins (24), who originally signed for one year, suffered a lower leg injury while in Japan with the Blues in July but is expected to return for Otago in the middle of next month.
The trio added some potency to the Blues back three this season and coach Tana Umaga is keen to keep them together.
''Collectively this trio, along with Rieko Ioane, enjoyed an excellent season and we believe all will take another step forward in 2018 and beyond,'' Umaga said.
''Michael Collins came in
primarily as a utility player but quickly asserted himself as an excellent fullback in all aspects
of the game and is an important communicator, winning the club rookie of the year.''
''Matt Duffie took time to adjust from rugby league this year [but] has shown to be among the very best wings in the competition with his aerial skills and ability to beat players.
''Melani Nanai is a local Auckland player who has come through the club ranks as an exciting open field runner.''