Dack's Duplicate

• WWizards of the Coast2014-05-29
  Official Ruling
  Once dethrone triggers, it doesn't matter what happens to the players' life totals before the ability resolves. You'll put a +1/+1 counter on the creature even if the defending player doesn't have the most life as the ability resolves.
• WWizards of the Coast2014-05-29
  Official Ruling
  Dack's Duplicate copies exactly what was printed on the original creature and nothing more (unless that creature is copying something else or is a token; see below), except it will have haste and dethrone. It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.
• WWizards of the Coast2014-05-29
  Official Ruling
  Haste and dethrone are part of the copiable values of Dack's Duplicate. If another creature enters the battlefield as or becomes a copy of Dack's Duplicate, it will copy whatever Dack's Duplicate is copying and have haste and dethrone.
• WWizards of the Coast2014-05-29
  Official Ruling
  If Dack's Duplicate somehow enters the battlefield at the same time as another creature, Dack's Duplicate can't become a copy of that creature. You may choose only a creature that's already on the battlefield.
• WWizards of the Coast2014-05-29
  Official Ruling
  The +1/+1 counter is put on the creature before blockers are declared.
• WWizards of the Coast2014-05-29
  Official Ruling
  The ability of Dack's Duplicate doesn't target the creature.
• WWizards of the Coast2014-05-29
  Official Ruling
  If the chosen creature is copying something else (for example, if the chosen creature is another Dack's Duplicate), then Dack's Duplicate enters the battlefield as whatever the chosen creature is copying. It will have haste and dethrone.
• WWizards of the Coast2014-05-29
  Official Ruling
  In a Two-Headed Giant game, dethrone will trigger if the creature attacks either player on the team with the most life or tied for the most life.
• WWizards of the Coast2014-05-29
  Official Ruling
  If the chosen creature is a token, Dack's Duplicate copies the original characteristics of that token as stated by the effect that put the token onto the battlefield. Dack's Duplicate is not a token, even when copying one.
• WWizards of the Coast2014-05-29
  Official Ruling
  You can choose to not copy anything. In that case, Dack's Duplicate enters the battlefield as a 0/0 Shapeshifter creature, and will probably die almost immediately, when state-based actions are next performed.
• WWizards of the Coast2014-05-29
  Official Ruling
  Any enters-the-battlefield abilities of the copied creature will trigger when Dack's Duplicate enters the battlefield. Any "as [this creature] enters the battlefield" or "[this creature] enters the battlefield with" abilities of the chosen creature will also work.
• WWizards of the Coast2014-05-29
  Official Ruling
  If the copied creature has in its mana cost (such as Grenzo, Dungeon Warden), X is considered to be 0.
• WWizards of the Coast2023-07-28
  Official Ruling
  Dethrone doesn't trigger if the creature attacks a planeswalker, even if its controller has the most life. The same is true if the creature attacks a battle, even if its protector has the most life.

Rulings via Scryfall