This issue was caused do to the manner and methods used to migrate the data.
The data was migrated using a date range and the delta migration included the end of the previous date range.
To avoid duplication of data in this setup, make sure that the date ranges don't overlap.
To make sure you get all the date from a particular date, it will be necessary to use a date in the past. For example. To get all data in a mailbox ending on the 10th of a month, the migration should be started on the 11th and only have the 10th as the end date. This will capture all selected data upto and including the 10th of the month, right up till 23:59.