Snippet for reproducing the Shift+D in a node graph editor in Blender

blender_duplicate_node.py

import bpy

def duplicate_node(n1, node_group=None, duplicate_links=False):
    """
    Duplicate a node (without duplicating its links)
    @param n1 The node to duplicate
    @param node_group (optional) Group in which the node must be duplicated
    @param duplicate_links Whether to keep input links or not
    @return the new duplicate node
    """
    if node_group is None:
        node_group = n1.id_data
    n2 = node_group.nodes.new(type=n1.rna_type.identifier)
    for in1, in2 in zip(n1.inputs, n2.inputs):
        if hasattr(in1, 'default_value'):
            in2.default_value = in1.default_value
        if duplicate_links:
            for link in in1.links:
                node_group.links.new(input=link.from_socket, output=in2)
    for out1, out2 in zip(n1.outputs, n2.outputs):
        if hasattr(out1, 'default_value'):
            out2.default_value = out1.default_value
    for prop in n1.bl_rna.properties:
        prop_id = prop.identifier
        if prop_id in n1.bl_rna.base.properties or prop.is_readonly:
            continue
        value = getattr(n1, prop_id)
        setattr(n2, prop_id, value)

if __name__ == "__main__":
    # Example, assuming there is a node called 'Extrude Mesh'
    g = bpy.data.node_groups['Geometry Nodes']
    duplicate_node(g.nodes['Extrude Mesh'], duplicate_links=True)

(Tested in Blender 3.1)