What must be ensured if an identifier specified has been previously used in the file?

To ensure that the identifier specified has not been previously used in the file, it's crucial to make sure that the identifier field is unique or overlay data. This uniqueness is fundamental in many data management contexts, particularly when dealing with fields that are meant to uniquely identify records or entries.

If an identifier is reused in a file, it can lead to confusion and data integrity issues, as it could cause incorrect associations or overwrites of data. By confirming that the identifier is unique, you maintain clear and distinct records, allowing for proper data retrieval and management. Overlaying data can be a strategy employed if the identifier must be reused, but it still emphasizes the need for ensuring that the overall identification scheme remains coherent and well-structured.

Other choices imply actions that do not directly address the requirement for identifier uniqueness. While removing all instances of the identifier might prevent duplication, it would be impractical and potentially harmful if those instances contain important data. Shortening an identifier name does not guarantee uniqueness and may create new conflicts. Confirming there are no errors in the file is important for overall data integrity but does not specifically address the issue of identifier uniqueness. Therefore, ensuring the identifier field is unique or overlaying data is the most effective and relevant approach.