Stunned is a binary state. A Unit is Stunned or it isn't. 1. A Stunned Unit can not be Stunned again. Example: Eclipse Herald has the ability "When you stun an enemy unit, ready me and give me +1 [M] this turn." Its controller plays a spell that reads "Stun a unit." They may choose a unit that's already stunned, but if they do, Eclipse Herald will not trigger. 2. Stunned Units lose the Stunned status during step 3d of the end of turn cleanup.
§ 423.1.a Core Rules PDF (officieel)
§ 423 Stun
§ 423.1 Stunning is the act of selecting one or more Units on the Board and rendering them Stunned.
