At the moment, if you don't use email address as a unique identifier, A/B tests can be problematic. For example, you have a family of 4 in the database, and all share the same email address. At the moment, the system puts records into the A/B/Remainder buckets and then de-dupes at time of send, rather than at time of creating the test.

This means that person 1 could be in bucket A, person 2 in bucket B and persons 3&4 in bucket C. We have quite a lot of situations with shared email addresses, which means we cannot use this testing optimally. It's quite embarassing when an email address receives both versions of a test email.