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.

An Aura spell requires a target, as defined by its enchant ability.

A resolving copy of a permanent spell becomes a token. That token isn’t “created” and won’t interact with abilities that care about tokens being created.

If the spell that’s copied has an X whose value was determined as it was cast, the copy will have the same value of X.

The copy will have the same targets 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.

The copy is created on the stack, so it’s not “cast.” Abilities that trigger when a player casts a spell won’t trigger.

If the copied spell divides damage or distributes counters among a number of targets, the division and number of targets can’t be changed. If you choose new targets, you must choose the same number of targets.

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.

Shiko and Narset, Unified’s last ability and the copy it creates resolve before the spell it’s copying. They resolve even if the original spell is countered before the copy is created.

If the second spell you cast targets a permanent or player, you must copy that spell, even if it won’t be possible to choose legal targets for the copy. You can’t choose not to copy it in order to draw a card.

Spells that were cast before a permanent with flurry count. If that permanent was the first spell you cast that turn, the next spell you cast that turn is your second spell.