How are you deriving that the email is delivered twice? is it through logs or are you checking the inbox? There is helpx document on the same as below. we also faced the same issue but we are looking at logs to derive that.
we are on the same boat and Adobe support didnot give proper response for the ticket I raised and said we need to look at OOTB broadlogs where I submitted my archived broadlogs as we done keep them for long. I really dont understand why it matters for them, but on my side the updated date on logs is not on same day and we are not sure if the customer got the email twice.
It would be great if you figured out something on this.