Entity integrity requires that each value of a primary key be unique within the table, and that no NULL values exist. If a client application attempts to insert or update a primary key value, providing values that are not unique would breach entity integrity. A breach in entity integrity prevents the new information from being added to the database, and instead sends the client application an error.
The application programmer should decide how to present an integrity breach to the user and enable them to take appropriate action. The appropriate action is usually as simple as asking the user to provide a different, unique value for the primary key.