When is the appropriate time to activate "repeat" on a chain, and can an opponent react before the repeat effect goes off?

Ruling: Repeat is an optional additional cost paid during the pay costs step of playing a spell. It creates one spell and one chain item, so there is no opportunity for your opponent to react between the initial effect and the repeat effect.

Sequence:

• When playing the spell, during the pay costs step, choose whether to pay the base cost or add the Repeat cost
• If Repeat is paid, the spell goes on the chain as one item that will execute its effect twice
• The opponent can only respond once the spell is on the chain, same as with any non-repeated spell

Nuances:

• Repeat can only be paid once per spell
• Playing a spell with Repeat still only counts as playing the spell once