I suppose with an incrementing system, you could doll out the search space more efficiently. I imagine the more hash power you have, the more likely any two engines could randomly try the same nonce. (Assuming they were working off identical blocks). But it could be that there's a relationship between the way difficulty ratchets that keep the chances of testing the same nonces twice somewhat constant relative to the chance of finding a block.

Even if you're working on the same block, it's never the same work for the processor, the timestamp, order of transactions, number of transactions, and the coinbase all make the work unique.