Card Duplication is a save corruption glitch that lets you gain extra copies of any card you already own.

The basics of how it works is, when the game is saving, it saves "copies you own of each card that are not currently in decks", followed by your decks.

So if you remove Dugtrio and Computer Search from your starting deck, you'll have 1 copy not currently in decks. Then, if you interrupt the save process at just the right time, it doesn't reach overwriting your deck, so your deck will still have a Dugtrio and Search as well. And whatever you added to fill the gaps, will be gone.

Trying to duplicate trainer cards, it's about a quarter-frame-perfect trick. The fadeout on reset on Game Boy Player (and emulated in GSR) makes it particularly different to time correctly.

The basic plan is to remove some key cards (like Dugtrio, Oak, PlusPower, Search), then have the reset happen just as you press A. If you save too late, your cards won't change at all. Sometimes, you'll get a partial dupe, which if too early, would likely mean the cards you added are gone (since Grass cards get their quantities written earlier than Fighting/Trainers do) but nothing is duplicated. If you save too early, obviously the save will complete and you'll just have your 1 copy outside of your deck.

Original work not done by Raagentreg