Dack's Duplicate, and Felidar Guardian

Prerequisites

• Dack's Duplicate in hand
• Felidar Guardian on the battlefield

Steps

1. Cast Dack's Duplicate by paying its mana cost, choosing to have it enter the battlefield as a copy of Felidar Guardian When Dack's Duplicate enters the battlefield, it triggers, blinking Felidar Guardian
2. When Felidar Guardian enters the battlefield, it triggers, blinking Dack's Duplicate
3. Choose to have Dack's Duplicate enter the battlefield as a copy of Felidar Guardian
4. Repeat from step 2

Results

• Infinite LTB
• Infinite ETB