Duplicate addresses can occur when the mapped fields are missing or are different on the linked records. For example, mapped fields such as Region or Country that exist on the Raiser's Edge preffered address record but are missing from Core record can cause duplicates.
Users will also notice a new address record created when the address in "ON" and the address in Raiser's Edge appear to match. This can occur if the address is shared between multiple records and you only process one record. Processing only one or some but not all of the available shared records in Connect RE will create a new address record that isn't shared. If you process all records that share that address it will leave the original address alone. Once users process the remaining records that share the address the sharing will be added.