A "duplicate key" refers to a situation where two or more entries in a database or data structure have the same identifier or key. This can lead to confusion and errors, as the system may not know which entry to reference. In databases, keys are essential for maintaining unique records, and duplicates can compromise data integrity. In programming, handling duplicate keys often involves implementing constraints or checks to prevent their creation. For example, in a relational database, a primary key constraint ensures that each record is unique. Proper management of duplicate keys is crucial for efficient data retrieval and organization.