Movie Doppelgangers - A List

What is the deal with all the movie pairs - 2 movies out within a year with essentially the same or very similar plot or theme.  When I began to think about these two Snow White stories (review below) - I started making a list of all the movies with a doppleganger.  The question is which is the real deal and which one is a copy.  I think I have concluded that one can always be deemed to be better and one will always be an inferior copycat.  Though, sometimes - its a pretty close call.  I'm sharing my list - it got kind of long - as I got serious about it.    The real deal first - followed by its doppelganger.

1982:     Tootsie - Victor/Victoria - Best. Guy. In. Drag. Ever.
86/87:    Short Circuit - *batteries not included - Johnny 5
88/89:    Oliver & Company - All Dogs Go To Heaven - Billy Joel. Huey Lewis.
1989:     K-9 - Turner and Hooch - Sorry, Tom - saw K-9 on my first date - sentimental real deal
1989:     Like Father Like Son - Vice Versa - Dudley vs. Judge? 2 words: Kirk Cameron.
93/94:    Tombstone - Wyatt Earp - 134 mins
1995:     Rob Roy - Braveheart - Liam Neeson
1997:     Dante's Peak - Volcano - Remington Steele 007
97/98:    Prefontaine - Without Limits - Jordan Catalano
1998:     Armageddon - Deep Impact - Gazelle

1998:     A Bug's Life - Antz - Dave Foley. California Adventure.
98/99:    Saving Private Ryan - The Thin Red Line  - Steven Spielberg
98/99:    The Truman Show - EdTV - Good afternoon, Good evening and good night
01/02:    Treasure Planet - Titan AE - Disney
03/04:    Van Helsing - League of Extraordinary Gentleman - Wolverine! by hair over 007
03/04:    Finding Nemo - A Shark's Tale - Just keep swimming
2004:     Troy - Alexander - Sorry Jordan Catalano, Brad Pitt - Brad Pitt - Brad Pitt
2004:     Chasing Liberty - First Daughter - Matthew Goode. Jeremy Piven.
05/06:    Capote - Infamous - Phillip Seymore Hoffman
05/06:    Madagascar  - The Wild - You got to move it.  Move it.
05/06:    Sky High - Zoom - Kurt Russell. Linda Carter!

05/06:    Garden State - Elizabethtown - Sorry Cameron Crowe - I  love you, but JD killed it.
2006:     Open Season - Over the Hedge - Ashton Kutcher
2006:     The Prestige - The Illusionist - Wolverine vs Batman with Magic
2010:     Killers - Knight and Day - by a hair  - Tom sort of creeped me out
2011:     Friends with Benefits - No Strings Attached - JT. Flash mob.
2012:     Snow White and the Huntsman - Mirror Mirror - Close call - serious and earnest wins!

Why does this happen?  My guess is that plots get pitched to multiple studios and multiple studios green light the same ideas?  Its an intersesting phenom.  One I'm sure will continue.

What movies and their clones did I miss?