First confirm that if the delivery had only one Broadlog for the recipient in question.
If yes, then search for the email address in the MTA child logs. If there is more than one occurrence, then review the status of each attempt.
In most cases, it is either attempted multiple times as we received bounce message back in every try, until we succeeded last.
Or if it is only sent once per the MTA Child logs as well, then from the actual deliveries pick a sample URL (tracked) from each message, and compare the delivery ID from the parameters list of all, and see if it is actually same.
If the message was "Technically" sent only once from Campaign, then the problem is usually at ISP's end.
In some rare occasions, it happens that even though we receive only one success response, ISP could have delivered the message more than once.
If the MTA child logs indicate multiple sends, under all other normal conditions, then it may need further investigation.