This Globe article about similar ad campaigns for different clients is interesting not because it offers insight into how two agencies can come up with the same idea, but because it can't. Short of defaulting to nasty accusations that would be difficult to prove and would likely be untrue anyway,* how the hell do you explain it?
The interviewees in the article conclude, "Hey, some ideas are just in the air" and the writer of the article doesn't seem to have much of an opinion about it either way.
Now, such a thing has never happened to me, and I can only imagine that the creatives involved are honestly mortified. My personal bias is that these "in the air" ideas tend to be okay ideas that have great or powerful executions – witness putting a "baby on board" sign on a hearse. Chances are, it's work that hasn't really been challenged as thoroughly as it could be, because it's work that could serve equally well for another client in the same category. And to me, a good ad is specific; it says something about that client that no one else can say. Can you imagine Microsoft doing Think Different or Labatt doing The Rant?
Even when it's cool and award-winning, a generalized statement that anyone can say (in other words, a cliché) is still, well...
Which makes me think that, in each case, more work could have been done.
*While individuals can be dumb enough to rip off work to make their portfolios artificially better, no agency or creative director I've ever met would think of doing such a thing, let alone allow it. It would end careers and drive away clients. No one campaign idea is worth that. No single idea is that good.