I think that is expected as duplication produces redundancy, and redundancy is certainly a selective advantage that can lead to more complex organisms. If you have one copy of a gene and it mutates (most likely a null mutation or a negative one) the organism might be SOL. If it has two copies, it might be able to make enough protein to function. If it has a hundred copies, one bad mutation won't make it any worse for the wear. It can keep "trying" new mutations on for size, and eventually one might be beneficial resulting in a trait that will be selected for.
The same can be said for redundancy on a macro-scale. The plesiomorphic arthropod had numerous homonomous segments that all basically had the same functions. It could continually "change" these appendages through the accumulation of mutations. The majority of these undoubtedly resulted in a useless segment, but some of them might have been advantageous. Thats how we arrived at such a wide variety of appendages on the decapoda (pincers, walking legs, swimming legs, uropod, etc.)