A New Jersey man killed his father and his grandmother — and then threw a party, prosecutors said yesterday.

Dwayne Flourney, 26, stabbed his grandmother, a retired schoolteacher, at the family's home on a tree-lined, suburban Maplewood street Saturday afternoon, the Essex County Prosecutor’s Office said.

When his dad arrived home later in the afternoon, Flourney fatally knifed him, too, authorities said.

He then allegedly hid the bodies, stashing grandmother Sandra Flourney, 76, in the trunk of a car parked in the home’s garage and leaving his father, Brian Flourney, 52, in a chair on the sun porch, covered by a sheet.

Later, he invited pals over to party.

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