Occasionally I’ll see a common theme from two unrelated sources and I’ll wonder: Did the makers of movie/TV show X take their inspiration from earlier movie/TV show Y? For example, was Lost In Space (a family journeying through space in a saucer-shaped craft) inspired by the Twilight Zone episode Third From the Sun (a family steals a saucer-shaped spacecraft to escape an upcoming atomic war)?
Any other examples where you’d swear the writers were influenced by an earlier appearence of a theme?
I’ve been wanting to ask about The Three Amigos, A Bug’s Life, and Galaxy Quest. I’ve been told they’re all based on The Seven Samurai, but I haven’t seen that one. The samurai aren’t actually performers/actors, are they?
A lot of the Family Guy plots are very similar to some Simpsons episodes, enough that I think the entire series is an attempt to rekindle the Simpson humor (which has obviously declined since Groening left to work on Futurama).
Jennifer Connelly has a scene of her standing on a foggy pier in The House of Sand and Fog that is eerily similar to earlier scenes in both Requiem for a Dream and Far Harbor yet all three directors swear that it was purely coincidental and that none of them watched the other films.
The Magnificent Seven is a more-or-less direct remake of The Seven Samurai if that helps.
That’s actually Dark City, not Far Harbor. And Dark City came first, so it couldn’t have borrowed it from the other two movies. 
Since Memento, there have been several other films featuring characters with some form of anterograde amnesia.
I can believe that Dory in Finding Nemo was inspired by the common belief (popularized by Ani DiFranco) that fish have no memories. However, every time I see an ad for 50 First Dates I imagine a pitchman saying “It’s The Wedding Singer meets Memento!”
ahh and didn’t Dana Carvey have a movie Clean Slate before Memento?!?!?!
I’d swear that Stephen King was thinking of Watership Down and The Lord of the Rings when he wrote The Stand.
After all, the first half of his book is where various someones step up as a unexpected leaders to herd a bunch of hlessi… er, stragglers to the Promised Land. And then one theme melts into another as the new colony decides to send some volunteers to the Enemy to do mighty deeds.
Or maybe King just read a lot of Joseph Campbell. Who knows?
Ken Russell’s Performance – shamelessly cribbed from Ingmar Bergman’s Persona.
Mirai Ninja (Warlord) (Japan, 1988) visually very similar to Star Wars, with some plot elements intact-- plot elements which Lucas borrowed from Kurasawa’s Hidden Fortress, so I guess it’s a three-fer. (Should we bring The Wizard of Oz into it, just to be crazy?)
Twelve Monkeys – from the short film (more of a narrated slide-show, actually-- there’s only one scene with some subtle motion in it) La Jetee.
Sunset Boulevard, from Eric von Stroheim’s unfinished Queen Kelly.
Twelve Monkeys is an adaptation of La Jetee - that was credited.
Another film inspired by The Seven Samurai is Roger Corman’s Battle Beyond the Stars. But trust me, this is not a film you want to spend money on.
The first time I saw Titanic at the movies I insisted to my partner that I had seen the “winning tickets on the Titanic” bit before. I figured Twilight Zone but I have never tracked down the source of my memory.
The Simpsons episode “Bart’s Comet” looks like it was based on “Deep Impact,” which was not released until three years after the episode aired. . .
Having bought the DVD just the other day, I’ll toss out that I’m one of those who believe The Lion King was based on/a rip-off of Kimba: The White Lion.
There was a Simpsons episode where Homer teams up with Mel Gibson to remake Mr. Smith Goes To Washington–this time as a gory action movie. The big joke in the sequence has Mel goring someone to death with an American flag as a parody of how tastless and over-the-top Homer’s movie is.
…The thing is, this aired several months before Mel pulled the same stunt, perfectly seriously, in The Patriot.
I strongly disagree.
“But, the Varda says ‘That which is not organic-’ "
" ‘Shall not destroy that which is.’ I know.”
“Now, Zaydar wears his left foot.”
“Lazuullliiiiiiiiiiii !”
An actual space cowboy, complete with stetson.
A spaceship with breasts
And starring, John boy!
This is definitely an MST3k, so-bad-it’s-good film.
LOST IN SPACE always struck me as being based on THE SWISS FAMILY ROBINSON; even believe it may have been originally called SPACE FAMILY ROBINSON (or maybe that was an unrelated comic).
Sir Rhosis
There were a number of X-Files episodes which seemed to take inspiration from past films or TV shows. Two that stand out in my memory are: the episode with the deformed killer twin (a la Basket Case ) and the chicken cannibal episode which ends with
the bad guy’s mask being pulled off, revealing the Sheriff
which was clearly inspired by Scooby Doo.
They are, as far as film is concerned, the archetypal “reluctant heros” and
Kikuchiyo, Toshiro Mifune’s character is an impostor.
And while we are at it, it is a wonderful film, one of my all time favorites (If you don’t mind 206 min of b&w subtitled japanese 
).
A Bug’s Life, Three Amigos, and Galaxy Quest are all a combination of two basic storylines.
First, the Seven Samurai/Magnificent Seven story: embattled villagers hire a group of mercenaries to defend their town against the bad guys. (Three Amigos spoofs Magnificent Seven explicitly in places, if I recall correctly.)
Second, what you might call the Brave Little Tailor storyline: cowardly wimp is mistaken for someone brave and powerful (you might also call it the Danny Kaye/Bob Hope storyline, since they both played that role more than once.)