BUG #15159: Duplicate records for same primary key

The following bug has been logged on the website:

Bug reference: 15159
Logged by: Sandhya KS
Email address: sandhya(dot)k_s(at)nokia(dot)com
PostgreSQL version: 9.3.14
Operating system: WindRiver
Description:

For 9.3.14 version, in customer environment we are facing the following
issue. Query to fetch the record for a particular primary key is displaying
one record. But when trying to display the duplicate records of that
table(sample command: SELECT <primary_key>, COUNT(*) FROM users GROUP BY
<primary_key> HAVING COUNT(*) > 1) is showing that there are 2 records for
the same primary key.

One more strange this is, if we delete all the records related to this
primary key, then one more primary key is showing up to be having duplicate
records. Checked the triggers as well: on delete no operation is being
performed on this table.

Questions:
1. Primary key is created to have unique records, then how two records are
being created/inserted?
2. After delete of malicious record, how other record is showing up to be
having duplicate records? Am I missing anything to be checked ?