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Lee Noble was sent off for the second time in a month as Dartford crashed to a ninth straight league defeat on Saturday.
Tony Burman's side had taken the lead at Alfreton, Elliot Bradbrook scoring from the penalty spot after Danny Harris was tripped by Darren Kempson.
But Noble was red-carded for an act of dangerous play which left Town's Connor Franklin with a nasty head injury.
That followed his dismissal against Gateshead on November 23.
Alfreton took full advantage by scoring twice in seven minutes after the interval.
Paul Clayton netted both goals, with Dan Bradley providing the assists.
Dartford still posed a threat on the break and Adam Birchall twice went close to an equaliser, before new signing Ishmail Kamara came off the bench for his Darts debut.
But there was to be no comeback and results elsewhere in Skrill Premier mean the Darts will be four points from safety at Christmas.