1. Uniqueness of data records is the degree to which data records occur only once in a data file.
   
2. Uniqueness of the mapping of an object in a data file is the degree to which objects (in the real world) occur as one data record in a data file.
   
3. Uniqueness of primary keys is the degree to which primary keys occur only once in a data file.

In general, double data records should be deduplicated.

1. This article appears twice in a data file under the same article number. This is easy to detect by checking on double article numbers.
   
2. This article appears twice in a data file under a different article number. This can only be discovered by comparison of other data elements than the article number.
   
3. Two different articles appear under the same article number in a data file. This can only be discovered by checking double article numbers and then compare other data elements than article number before deduplicating.

DAMA NL (2020). Dimensions of Data Quality (DDQ). Research paper. https://www.dama-nl.org/wp-content/uploads/2020/09/DDQ-Dimensions-of-Data-Quality-Research-Paper-version-1.2-d.d.-3-Sept-2020.pdf