Specifically what is the error?
psql (9.1.5)
Type "help" for help.
postgres=# select pg_start_backup('hotbackup',true);
pg_start_backup
-----------------
61/B000020
(1 row)
postgres=# select pg_start_backup('hotbackup',true);
ERROR: a backup is already in progress
HINT: Run pg_stop_backup() and try again.
postgres=#