Mailwizz only retries sending if the response from the smtp end says that sending didn't work out.
Then the status of the email is marked as temporary error and queued for later sending. If we get same error 3 times, then we mark the email as an error and we log the reason why it errored out.
You should look in the list delivery reports for that particular email address and see if in the end sending was successfully or not.
Secondly, what delivery method are you using?
Thirdly, it's just him that received the message 3 times?
Fourthly, what mailwizz version?
Test this again and while the campaign is sending, take a look in backend -> misc -> campaigns delivery logs, at the message column.
One more thing, when you say they receive the email 3 times, does this also gets reflected in the campaign stats and the number of emails a customer sent so far?