Doublecast

• WWizards of the Coast2018-07-13
  Official Ruling
  If you cast one Doublecast after another, you’ll copy the second Doublecast. Each resolves, creating two delayed triggered abilities. The next spell you cast after that will be copied twice. If that spell is also a Doublecast, the following spell will be copied three times, and so on.
• WWizards of the Coast2018-07-13
  Official Ruling
  You can’t choose to pay any additional costs for the copy. However, effects based on any additional costs that were paid for the original spell are copied as though those same costs were paid for the copy too.
• WWizards of the Coast2018-07-13
  Official Ruling
  If the spell that’s copied has an X whose value was determined as it was cast, the copy has the same value of X.
• WWizards of the Coast2018-07-13
  Official Ruling
  Doublecast’s ability will copy any instant or sorcery spell, not just one with targets.
• WWizards of the Coast2018-07-13
  Official Ruling
  A copy is created even if the spell that caused Doublecast’s ability to trigger has been countered by the time that ability resolves. The copy resolves before the original spell.
• WWizards of the Coast2018-07-13
  Official Ruling
  The copy that Doublecast’s ability creates is created on the stack, so it’s not “cast.” Abilities that trigger when a player casts a spell won’t trigger.
• WWizards of the Coast2018-07-13
  Official Ruling
  The copy will have the same targets as the spell it’s copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. The new targets must be legal.
• WWizards of the Coast2018-07-13
  Official Ruling
  If the spell that’s copied is modal (that is, it says “Choose one —” or the like), the copy will have the same mode or modes. You can’t choose different ones.

Rulings via Scryfall