Lot of people buy multiple games in one month. Check my sig for proof of this. I also bought Medal of Honor Warfighter recently but traded it in.
I usually save money in the summer when there aren't many good games getting released and keep it for October onwards so I can get all the games I want on day of release.
As for whether Halo 4 will skew Black Ops 2 sales, I doubt it. Halo has not been top dog at all this gen for sales. If Halo 4 is as good as all the reviews say it is, then Halo 5 COULD get closer to the next Call of Duty game.
Currently playing Dishonored, Borderlands 2 & Resident Evil 6. Next games I'm buying - Halo 4, Black Ops II, Hitman Absolution
Will it impact BO2 sales?...Eh, not really or at least not significantly. I think some people may indeed switch their preorders, but all-in-all CoD will still outsell. It's mostly due to the multi-platform aspect of CoD
Now don't get me wrong, I'm absolutely 100% excited for this game and have it AND Black Ops 2 paid off. The whole "gamers won't buy 2 games in one month" thing is kinda silly though.
Who knows, Black Ops 2 is already the most pre-ordered game in the franchise and considerably more preordered when you take all platforms into consideration. Would it have sold significantly more if Halo 4 wasn't coming out? Don't know.
If anything I think Black Ops 2 hurts Halos sales. Had this came out more then a week before COD I woulda bought Halo but what's the point now? Once Duty comes out I'd never play Halo again. I'd only get Halo to hold me over. I'm sure I'm not the only one.