Can you stun units that are already stunned?

Ruling: No, you cannot stun units that are already stunned. However, you can still target stunned units with stun effects.

Nuances:

• You can target already-stunned units with stun effects, but the stun won't apply
• Effects that trigger on stunning a unit (like Leona) will not trigger when the stun effect resolves on an already-stunned unit