remove-duplicate-from-db.php

$servername = 'localhost';
$username = 'drupal';
$password = 'drupal';
$database = 'drupal';
$table_name = 'employees';

try {
  $conn = new \PDO("mysql:host=$servername;dbname=$database", $username, $password);
  // set the PDO error mode to exception
  $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
  $query = $conn->query('select emp_email, emp_name from ' . $table_name);
  // Returns all rows with key based on first column i.e. emp_email
  // We are assuming uniqueness based on the email address only.
  // This means two employess can have same name but not same
  // email address and thus bitwise '|PDO::FETCH_GROUP' to group
  // the result based on email.
  $results = $query->fetchAll(PDO::FETCH_COLUMN | PDO::FETCH_GROUP);
  foreach ($results as $mail => $names) {
    // If names are more than 1, this means this email has duplicate records.
    // Delete the rows
    if (count($names) > 1) {
      // Delete all records based on email.
      $statement = $conn->prepare("DELETE FROM $table_name WHERE emp_email=?");
      $statement->execute([$mail]);

      // Insert the record.
      $statement = $conn->prepare("INSERT INTO $table_name VALUES('$names[0]', '$mail')");
      $statement->execute();
    }
  }

  // Close the connection.
  $conn = null;

} catch (\PDOException $e) {
  echo "Connection failed: " . $e->getMessage();
}