Twincast + Reverberate

Steps

1. Cast any instant or sorcery spell, or wait for an opponent to cast one.
2. Holding priority, cast Reverberate by paying {R}{R}, targeting that spell.
3. Cast Twincast by paying {U}{U}, copying Reverberate, with the copy targeting Reverberate.
4. Resolve the copy of Reverberate, copying the original Reverberate, with the copy targeting the original Reverberate.
5. Repeat step 4.