You'll need the following cards:
Library of Leng
Darkpact
Disrupting Scepter
Ring of Ma'rûf

Duplication portion of the setup:
You have Disrupting Scepter in play. You have Library of Leng in Play. You have Darkpact in hand. You have the card you want to duplicate in hand.
1: Activate Disrupting Scepter and discard the card you want to duplicate. Choose to put it on top of your library with library of leng.
2: Activate Darkpact and target the opponent with it (it's weird, but it does ask this).
3: Win the duel.

The result is that your chosen card will become the opponent's ante and you will receive a duplicate of it.

Ring of Ma'rûf allows you to get any card you want into your hand, even if you don't own it. Use this in combination with the duplication portion of this setup to get that card permanently.

It helps to have some sort of lockdown going to make sure you can do this safely. Lifetap and Magical Hack helped me in the past with this. Just pick on weak opponents too.

4:
Enjoy your ...
20x Ancestral Recall
20x Black Lotus
15x Berserk
5x Ball Lightning
... Deck (there's an in game special thing that allows you to break the 4 card limit)

Technical aspect about dealing a lot of damage, but involves a spoiler, won't matter in normal duels probably

OK that spoiler is hilarious.

Not a special thing in the game, the game follows the old rules that have no 4 of limit.

This is fantastic btw and I need to stream that game someday