John Williams has done it before the Imperial March from The Empire Strikes Back (and just about every subsequent movie in the series) is suspiciously similar to Chopin's Funeral March, with elements of Gustav Holst's Mars: Bringer of War movement from The Planets (the metre is changed and the order of the themes is mixed up a bit, but the similarities are fairly striking).
The main theme of the series played during the title crawls combines elements from Dukas' "The Sorcerer's Apprentice" and Tchaikovsky's "Marche Slave".The Phantom Menace uses a knockoff of the third movement of Dvorak's ninth symphony in the lightsaber battle at the end.James Horner's four-note "danger motif" is put in freakin' EVERYTHING ( Willow (starting at 0:34), Enemy at the Gates (0:32), Troy, Avatar (5:24), Star Trek II (1:10), etc.), but it isn't even something he came up with! It's originally from Sergei Rachmaninoff's first symphony.
Horner was known for reusing cues in his different scores, nearly to the point of self-plagiarism.Several parts of James Horner's Stealing the Enterprise, Battle in the Mutura Nebula, and Genesis Countdown in Star Trek II and Star Trek III was later reused in his own score for the The Rocketeer and parts of the score was reused in Walt Disney World's Fountain of Nations in Epcot.The song "Let's All Sing Like the Birdies Sing" in Disneyland/Disney World's Enchanted Tiki Room is suspiciously similar to Offenbach's "Barcarolle" from The Tales of Hoffmann (also called "Belle Nuit" from Les Comtes d'Hoffmann).The two 5 note phrases that open National Treasure 2: Book of Secrets are copied directly from Luke and Leia's theme from Return of the Jedi.