BUG #6300: duplicate key value violates unique constraint

The following bug has been logged online:

Bug reference: 6300
Logged by: Tigran Mkrtchyan
Email address: tigran(dot)mkrtchyan(at)desy(dot)de
PostgreSQL version: 9.0.4
Operating system: Linux x86_64
Description: duplicate key value violates unique constraint
Details:

Hi,

our application uses PostgreSQL as a backend and heavily
relays on transactions. We never check constrains in our application as DB
does is better than we can an in an atomic way. This works very-very good!
Nevertheless the logfiles are full of 'duplicate key value violates unique
constraint'. I have two questions:

1. why it logged with ERROR level as it clients application behavior.

2. is there a way to suppress it or could you add a way to suppress it.

Thanks a lot,
Tigran.