If I'm Duchene's agent, I'm using Couture as the comparable.
Couture is 30. Duchene is 28. Couture signed an 8-year, 64 mil (8 mil AAV) extension when he was 29.
Couture has 9.5 seasons in the league. Duchene has 10. Couture has 663 career games to Duchene's 727.
Couture has 4 seasons over 60 points, including last year. Duchene has 3 seasons over 60 points, including last year.
Couture has scored 20 or more goals 8 times. Duchene has done so 7 times.
With only a year passing since Couture signed, I don't think there will be much, if any, inflation. Plus as Duchene can only sign for a maximum 7 years, to give him the same 64 million, it comes out to a 9.14 million cap hit.