import bmesh
import bpy

bm = bmesh.new()
bm.from_mesh(bpy.context.object.data)

bm.faces.ensure_lookup_table()
bm.verts.ensure_lookup_table()

new_face = bm.faces[0].copy()

bm.verts.ensure_lookup_table()
bm.faces.ensure_lookup_table()

assert(new_face.verts[0].index != -1)