Theoretical Duplication is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever a nontoken creature an opponent controls enters this turn, create a token that's a copy of that creature.

Complete guide

Theoretical Duplication is a clever blue instant that turns your opponent's recruitment efforts into your own gain. By casting this spell, you set up a surveillance state for the rest of the turn, ensuring that any new physical presence on their board is matched by a token on yours.

What It Does

When you cast Theoretical Duplication, it creates a lingering effect that lasts until the end of the current turn. During this window, whenever a nontoken creature enters the battlefield under an opponent's control, a triggered ability goes on the stack to give you a copy of that creature. This works regardless of how the creature entered, whether it was cast as a spell, returned from a graveyard, or put onto the battlefield by an effect.

Resolution Step by Step

First, you cast Theoretical Duplication and it resolves, moving to your graveyard. From that point until the turn ends, the game watches for creatures. If an opponent's creature enters, the delayed trigger fires. When that trigger resolves, you create a token. This token copies all the printed characteristics of the creature, including its name, types, mana cost, and abilities. It does not copy temporary buffs, counters, or equipment attached to the original creature.

Common Mistakes

One common error is thinking this spell copies token creatures; it explicitly only triggers for 'nontoken' creatures. Another mistake is timing: the spell must be fully resolved before the creature enters the battlefield. If you cast this in response to a creature spell, the creature spell is still on the stack. Once Theoretical Duplication resolves, the creature spell will eventually resolve and enter, triggering your new effect.

When to Play It

The best time to play Theoretical Duplication is when you anticipate multiple creatures entering at once, such as during a resolved mass-animation spell, or in response to a powerful creature spell on the stack. Since the effect lasts for the whole turn, it is also effective if an opponent is known for playing multiple creatures in a single main phase.

Interaction examples

Direct Mirroring

Theoretical Duplication

If your opponent casts a powerful creature, you cast Theoretical Duplication in response. After your spell resolves, their creature enters the battlefield. This triggers the effect created by Theoretical Duplication, and you receive a token that is an exact copy of that creature.

Counterplay Interactions

Theoretical Duplication

Counterspell

If an opponent tries to use Counterspell on Theoretical Duplication while it is on the stack, your spell will move to the graveyard without resolving. No delayed trigger will be created, and you will not receive any tokens when their creatures enter later that turn.

Strategic Card Draw

Theoretical Duplication

Arcane Denial

You can cast Theoretical Duplication to set up your board. If your opponent later tries to remove one of your newly created tokens with a spell, you can use Arcane Denial to protect it. You'll stop their spell, and both players will draw cards on the next upkeep, while you keep your copied creature.

Guide written with AI from the official text and rulings. Official English text prevails.