Let me explain what I mean by this. By unnecessary, I mean these movies had no reason to be made at all. Sometimes, bad sequels were part of a story (Matrix, Star Wars, etc.) These are sequels of great movies that didn't need to be made, and made the originals seem a little worse.
10. Be Cool (F. Gary Gray, 2005)
Sequel Of: Get Shorty (Barry Sonnenfield, 1995)
9. Grease 2 (Patricia Birch, 1982)
Sequel Of: Grease (Randal Kleiser, 1978)
8. Caddyshack II (Allan Arkush, 1988)
Sequel Of: Caddyshack (Harold Ramis, 1980)
7. Jaws: The Revenge (Joseph Sargent, 1987)
Sequel Of: Jaws 3 (Joe Alves, 1983)
6. Blues Brothers 2000 (John Landis, 1998)
Sequel Of: The Blues Brothers (John Landis, 1980)
5. Ocean's 12 (Steven Soderbergh, 2004)
Sequel Of: Ocean's 11 (Steven Soderbergh, 2001)
4. S. Darko (Chris Fisher, 2009)
Sequel Of: Donnie Darko (Richard Kelly, 2001)
3. The Godfather: Part III (Francis Ford Coppola, 1990)
Sequel Of: The Godfather: Part II (Francis Ford Coppola, 1974)
2. The Two Jakes (Jack Nicholson, 1990)
Sequel Of: Chinatown (Roman Polanski, 1974)
1. The Sting II (Jeremy Kagan, 1983)
Sequel Of: The Sting (George Roy Hill, 1973)