I'd summarize them as follows:
Volume 1 - biology of corals, system setup, disease, stony corals, clams
Volume 2 - soft corals, zoos, mushrooms, anenome, propagation
Volume 3 - as described previously, heavy system orientation
As far as duplication, half of volume 1 is probably updated in volume 3, which incorporates the latest trends. Both are still worth it since the stony corals and clams are unique to volume 1.
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