BUG #6706: pg_upgrade fails when plpgsql dropped/re-created

The following bug has been logged on the website:

Bug reference: 6706
Logged by: Maciek Sakrejda
Email address: m(dot)sakrejda(at)gmail(dot)com
PostgreSQL version: Unsupported/Unknown
Operating system: Ubuntu 12.04 LTS (3.2.0-25-generic x86_64)
Description:

Using the 9.2beta2 of pg_upgrade and for the upgrade-to cluster.

Ran into an issue upgrading a 9.1 cluster via pg_upgrade. I can reproduce it
consistently:

1. Create a 9.1 cluster
2. Run "drop extension plpgsql" as superuser
3. Run "create extension plpgsql" as non-superuser
4. Perform normal upgrade via pg_upgrade

The last step fails and I get the following error in
pg_upgrade_restore.log:

SELECT binary_upgrade.create_empty_extension('plpgsql', 'pg_catalog', false,
'1.0', NULL, NULL, ARRAY[]::pg_catalog.text[]);
psql:pg_upgrade_dump_db.sql:40: ERROR: duplicate key value violates unique
constraint "pg_extension_name_index"
DETAIL: Key (extname)=(plpgsql) already exists.