Let's say you are with Gaction A. They beat the goal, 36/36 and you get the Black Skull HQ, then you switch over to Faction B, they have not completed the event yet, you help them and finish the 36/36. Do you get the HQ prize twice or just a waste of time and gold?
If it's possible, my god the lower population of MW is screwed. Heavy gold players can join their mini factions just to get the prize again.