Your people must be more sociable than mine, because if we depended on hallway conversations to detect this sort of redundancy we'd be doomed. Rather, adding the sorts of communication outlets that tend to help support remote people also tends to increase communication among the entire team, not to mention that many forms (IM, blog, wiki) create a readily-accessible archive for future reference.

Also, I don't know how big your company is, but your scenario seems far more likely at a large company than a small one.