There aren't very many almost identical, but there are a lot of communities that have significant overlap.

  - Superuser
  - Serverfault
  - Database Administrators - 8.9 questions/day
  - IT Security - 6.9 questions/day
  - Healthcare IT - 1.1 questions/day

The stackexchange community is all about forking. Clearly Healthcare IT, IT Security, and Database Administrators could all be tagged questions in serverfault. But apparently serverfault.com/questions/tagged/heathcare didn't sit well with the 298 people who committed to the beta. Separating power-users from IT professionals (superuser vs serverfault)is certainly reasonable even though there is a large overlap of knowledge there.

Of course, without the forks, as an IT person there might be a question if your cryptography related question should go in cryptography.SE or serverfault. With a dedicated site the "correct" location for those questions is easier to determine.

But if we're going to talk about overlapping stackexchanges we have to look at those populated predominantly by programmers, because... well, they are the master forkers.

  - stackoverflow
  - Programmers
  - Code review
  - Theoretical Computer Science
  - Code Golf
  - Signal Processing 
  - Computational Science

I left out the language specific ones like Mathematica and TeX. I also omitted Software QA and Cryptography.

Apparently theoretical physicists and applied? physicists are unable to co-exist on one stackexchange.

Anyway, clearly the stackexchange community thinks that forking is the answer. I think tagging is superior and I think fragmenting the community gets fewer able eyes focused on questions. Forking doesn't really hurt google users trying to find answers to questions that have already been answered.

Even while not being a professional developer I would stick Code golf, code review, and Programmers into one stack exchange. I'd also put Signal Processing, Mathematica, Tex, and Compuational Science back in Stack Overflow and let the theoretical CS folks stay separate.

One for each question that is moved out of StackOverflow, of course.

Reddit has something similar with these:

http://reddit.com/r/netsec

(Check related subreddits)

And that is a perfectly valid reason for Google to do it as well and bash Facebook while they are at it.