Today I faced a situation where I saw a postgresql 9.2 database that had duplicated rows in pg_autid. I saw 6 roles while 3 of them were duplicated (3 uniques rolenames in total). I checked and the oid of the duplicates were the same oid as the originals. The solution was running vacuum on the table but I wanted to understand how this situation was created ? Why even with unique index on the rolname I saw duplicated values for rolname ?