Rule 423.1.a

Stunned is a binary state. A Unit is Stunned or it isn't.

• 423.1.a.1 A Stunned Unit can not be Stunned again. Example: Eclipse Herald has the ability "When you stun an enemy unit, ready…
• 423.1.a.2 Stunned Units lose the Stunned status during step 3d of the end of turn cleanup.

Official Core Rules