Even before either side does a first-pass review of their collected documents, they can easily identify which potentially-discoverable documents both sides already have in common. This process would be fast, inexpensive, and easy, and would allow new kinds of cooperation between parties.
Please take a look at Craig Ball’s informative blog post on deduplication and my comment. Craig explains why deduplication isn’t more widely used even though it makes ediscovery faster and cheaper, and he describes the main methods of deduplication. I add my view that reasonable counsel should agree on a bilateral deduplication method before any deduplication is done, and I include a few considerations about how counsel should use proportionality to determine what method is appropriate under the circumstances.