After running an A/B test, some Lead records appear to have received both Test and Winner.
In an A/B test, the sample size of the audience is sent the variants, and then for the winner: the entire audience minus the test audience is sent the Winning email.
So let's say that you received both Test and Winner. This means you have duplicate records in the database.
Marketo is designed to not send duplicate leads the same email at the same time.
So one of the duplicates is qualified for the Test, and then when the Winner is sent, it's the total audience minus the test audience, the Winner is sent to the duplicate record.
This above situation can apply to Lead Records as well, because of duplicates with the same email address, but also duplicate records that may have a FWD set up between them (record B emails FWD to Record A email address).
Because this is expected from the design decision, the resolution is to deduplicate your database.