The special event has been the same for two weeks now, yes, but it did count a second time towards the medal and achievement. I personally think this event is the best one they've had so far, so I'm okay with it repeating for a second week.
I often start in a match with only 3-4 people, but they're usually full, or nearly full, by the time the match ends. But yeah, I'm consistently seeing only about 150-300 people on the playlist at any given time.
I really enjoyed this Event as well. It's so simple, which is probably what makes it fun.
A big positive with the redundancy, though, is that at least we know it won't have the issue Gears of War 3 had. This same Event is awarding a second ribbon to the Event counter, so that if they reuse Events it won't keep players (especially people who miss Events or get into this game later) in limbo for the medal and achievement.