  
  
  
 That's because there are four middle-manager  whose manager is Warbucks Oliver.

verdammelt

To add more detail:

Because Warbucks supervises:

1. Scrooge who supvervises Cratchet

2. Bitdiddle who supervises Hacker

3. Bitdiddle who supervises Fect

4. Bitdiddle who supervises Tweakit

Each path is taken and is reported as a separate case.

revc

find all unique tuples (variable assignments of the body of rule), but just extract some element(s) from tuple, which leads to the duplicated responses.

;;;modified rule
(rule (wheel ?person ?middle-manager ?x)
      (and (supervisor ?middle-manager ?person)
           (supervisor ?x ?middle-manager)))
;;;output
(wheel (Warbucks Oliver) (Scrooge Eben) (Cratchet Robert))
(wheel (Warbucks Oliver) (Bitdiddle Ben) (Tweakit Lem E))
(wheel (Bitdiddle Ben) (Hacker Alyssa P) (Reasoner Louis))
(wheel (Warbucks Oliver) (Bitdiddle Ben) (Fect Cy D))
(wheel (Warbucks Oliver) (Bitdiddle Ben) (Hacker Alyssa P))

sicp-ex-4.65