ERROR: attempt to redefine parameter "postgis.backend"

[strk]

Upgrading, via extension, from 2.1.0 to 2.2.0dev:

[strk@cdb:/usr/src/postgis/postgis(svn-trunk)] regress/run_test.pl -v regress/tickets.sql --extensions --upgrade-path 2.1.0--2.2.0dev
Upgrade path: 2.1.0 --> 2.2.0dev
Creating database 'postgis_reg' 
Preparing db 'postgis_reg' using: CREATE EXTENSION postgis VERSION '2.1.0'
Upgrading PostGIS in 'postgis_reg' using: ALTER EXTENSION postgis UPDATE TO '2.2.0dev'
 failed (Error encountered altering EXTENSION POSTGIS: /tmp/pgis_reg/regress_log)
-----------------------------------------------------------------------------
createlang: language "plpgsql" is already installed in database "postgis_reg"
CREATE EXTENSION
ERROR:  attempt to redefine parameter "postgis.backend"
-----------------------------------------------------------------------------
Died at regress/run_test.pl line 1265.

[robe]

This is the same as #2382 I believe. though your title is better so I'll change the title on that one. At the time I thought it was a pgAdmin only issue.