Way too many to count. At least half of my GOG games catalogue is games that I already have either on a disc or on floppy discs, and some even on both. Also, GOG had a thing where if you had a Steam version of some game, you got the GOG version for free, further bolstering the catalogue.
As for console games (not counting Switch online):
The Legend of Zelda (NES/Virtual Console)
The Legend of Zelda II (NES/GBA/VC)
Super Mario Bros 2 (NES/GBA/VC)
Super Mario Bros 3 (NES/VC)
Teenage Mutant Ninja Turtles (NES/VC)
Super Mario World (SNES/VC)
Street fighter II Turbo (SNES/VC)
Secret of Mana (SNES/VC/Switch)
Secret of Evermore (SNES/VC)
Donkey Kong Country 1-3 (SNES/VC)
Wind Waker (Gamecube/Wii U)
Freedom Planet (PC/Wii U)
I probably forgot some games that I have at least double on consoles, but that's all that comes to my mind right now.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454 List as Google Doc: https://docs.google.com/spreadsheets/d/1aW2hXQT1TheElVS7z-F3pP-7nbqdrDqWNTxl6JoJWBY/edit?usp=sharing
The Steam/GOG key gifting thread: https://gamrconnect.vgchartz.com/thread/242024/the-steamgog-key-gifting-thread/1/
Free Pc Games thread: https://gamrconnect.vgchartz.com/thread/248138/free-pc-games/1/
