Let's say someone has brand, they declare them their 2 year keeper. Now that person gets Duncan. First of all, he can now only declare Duncan that one year. Second of all, it has little to do with what keeper is your best and 2nd best keeper, but when you acquire that keeper.
second of all, what if you have brand (the aforementioned 2 year keeper) and you trade for duncan, who's declared as a 2 year keeper, and you retain contracts. in a bit if a pickle, no?
Besides, I don't really envision a whole lot of keepers (especially upper tier keepers) slipping through free agency. Someone with said keeper will want to get value, either another top of the line keeper or to find someone to overpay (and thus give them a better shot at this years title) for the rights to that keeper.
I think we're trying to overanalyze things. Right now, the rules are pretty much set unless we find an oversight. Not that I'm being a stickler, or power hungry, but rules need to be defined so we have something to go by. This can't be debated upon forever.