title: PostgreSQL Unique Key column contains duplicate records
body: <p>I have a table in PG 9.6 with unique key constraint on it. I took the backup using pg_dump and restore it in PG12. While restoring I got error for the table mentioned with unique key constraint failed. After checked in PG 9.6, Column contains few duplicates.</p>
  <p>Here Question is</p>
  <ol>
  <li>How is it possible for containing duplicates in column with unique key constraint ?</li>
  <li>Also, how to fix this error in PG12 without deleting duplicates (PG 9.6 contains duplicates)?</li>
  </ol>