Now they say that it's 100% for neurons. Exact 100% number is also pretty sketchy from statistical standpoint. What class of problem is searching for duplicate

Now they say that it's 100% for neurons. Exact 100% number is also pretty sketchy from statistical standpoint. What class of problem is searching for duplicates or uniqueness among a set of billions of items? If it is a tractable problem, what is the most effective algorithm?