Blues are the champions
Blues were 2-0 down but goals from substitute Ryan Walton and Duffy secured the draw.
Manager Gareth Davies said: "'Im delighted. It's just reward for a special bunch of players. I'm proud of them all and they have got what they thoroughly deserve.
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Hide Ad"It's a proud moment for the whole club and one which will go down in the club's history."
Blues will be presented with the Division 3 Trophy after their home match with Forest on Saturday (2pm kick-off).
RUSTINGTON: McCarthy; Butcher, Bibb, Neal, Oakes; Duffy, Morten, Butt, Withers; Moss, Smart. Subs: Walton (Morten 30), Nally (Moss 65), Cox (Hellen 75), Tomlinson, Webb.
For reaction from Rustington captain, Russ Tomlinson, see Thursday's Littlehampton Gazette.
Click here for pictures from Newhaven v Rustington.