Distinct

Some SQL queries may produce duplicate results. In such cases, duplicate rows may be filtered out using the "DISTINCT" keyword in a static query. In a dynamic query, use the distinct() method.

// Force filtering of duplicate records in the result set.
$connection = \Drupal::database();
$query = $connection->select('my_table', 'mt');
$query->fields('mt', ['my_fields']);
$query->distinct()->execute()->fetchAll();

Note that DISTINCT can introduce a performance penalty, so do not use it unless there is no other way to restrict the result set to avoid duplicates.