Special thanks to the folks of the Goat Format War League server for their help with the article.
The vast majority of the time you will profit by playing Delinquent Duo as soon as possible. You draw it, check your Life Points, then drop it onto the board. Your opponent will have less information to work with while they choose which card to discard if you play Duo before playing anything else during your turn. In a vacuum, Duo is easy card advantage, and nearly every deck wants it. If Duo is in your opening hand, even better! Duo is a particularly strong hand disruption Spell but can be even more devastating against decks that rely on combos to function, across that spectrum from Goat Control to Panda Burn, and all the way to decks like Library FTK, Spell Economics OTK, etc. Duo grants a considerable advantage to the player that uses it first; don't let your Duo get hit by Duo if you can help it.
Is there any time it is appropriate to hold Duo for later? Turbo Chaos is particularly resilient to Duo, which is another incentive to play it if you have it. You don't want to wait for your opponent to draw into or get value out of their Thunder Dragons, Night Assailants, or Sinister Serpent. You might even hit a Chaos monster! Due to its knack for gaining card advantage, if they go first this matchup can force you to hold your Duo until they expend some of their cards. You don't want to waste a Duo on their Thunder Dragons, etc., you want it to have impact. Waiting for a known Sinister Serpent to finally hit the graveyard is a common gameplay rhythm. You should also hesitate to play Duo if you can't answer a Chaos summon and the opponent's Graveyard has a LIGHT or DARK in it, but not both.
Despite its power, Duo is the weakest of the Trinity pieces (the others being Pot of Greed and Graceful Charity, of course), as there are some cases where it is simply not a good draw. Your opponent could be topdecking, for example. If your Life Points are 3000 or below and you haven't seen an opponent's Ring of Destruction yet or have an open board against opposing beaters, you may hesitate to pay the 1000 LP cost. Ring + Black Luster Soldier - Envoy of the Beginning, Ring + Mobius the Frost Monarch, Ring + Chaos Sorcerer--all can end a game for you if you aren't careful with your Life Points by the late game. Topdecking a Duo is miserable in a deck like Warriors, who thrive on simplified game states and thus might not have an immediate use for it at that point in the game.
What if your opponent has only one card in hand? Is it worth it to activate Duo? That's more complicated and can be matchup-dependent but the answer is yes in most cases. Another way to look at it is: can you wait for your opponent to gain more resources? YuGiOh! is a fast paced game, despite Goat being a relatively slow format, so the answer there will generally be no, you cannot afford to wait.
When do you side out Duo, if at all? Don't side it out against Panda Burn, as the two cards combined that you remove from their hand would probably add up to over 1000 damage anyway--a single Just Desserts can inflict up to 2500 damage. Duo does suffer when going second against Warriors and Turbo Chaos. Even then, you have to ask yourself, is what I'm siding in more effective for the matchup than discarding two of my opponent's cards?
Magician of Faith can be absolutely devastating with Duo, so you'll want to set that play up if your deck supports it. I would not want to be across the table from Warriors with double Duo as my main plan for the game, however. They will try to empty their hands first to power up their Blade Knights, rendering Duo useless, and they can also negate the FLIP effect I hope to resolve in the first place. The double Duo play is more likely to come up with Turbo Chaos on the other side of the table, and is how one can break a stalemate of passed turns whereby both players accumulate cards in hand without significantly altering the field.
If you play your Duo before summoning your Kycoo the Ghost Destroyer, you may gain the opportunity to banish any FLIPs, Chaos fodder, or Sinister Serpents that hit the graveyard. Don Zaloog and Spirit Reaper alongside Duo can turn an opponent's uphill battle into an impossible one. Pressure goes well with Duo.
Trap Dustshoot, despite being an excellent hand disruption card in its own right, often conflicts with Duo. You can Dustshoot your opponent to learn their hand contents before deciding to play Duo, but you could lose your Duo to Card Destruction or another Duo while you try to set that play up. The amount of risk varies with board state and matchup, obviously if your opponent's Duo is in grave and there's no Set monster on your opponent's side of the field, feel free to set up the two for three trade that is Dustshoot-into-Duo. Assuming a turn one opening with both cards in question, it is best to play Duo first.
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