By default, the first record delivery will include a full set of records from the collection, including records you have already received. For every subsequent record delivery, the default is to include only records with changes (updated, added or deleted). Also, if the same item/OCLC number is in multiple collections and the collections are set to different delivery frequencies, you could get the same record multiple times.
To change the defaults for record output, go to the collection's Holdings and MARC Records tab. Make sure MARC record delivery is enabled, then navigate to the Record Delivery tab.
If you want to receive copies of MARC records your library has already received, including records already held, you'll want to choose the following:
If you do NOT want to receive records your library has already received:
Either way, we recommend keeping Record Output on the "Include records already held in WorldCat" setting for all subsequent record deliveries.