Linux: ps output shows multiple instances of PostgreSQL running on the same port

Following a recent database crash, I encountered the following when doing
debug:

[root(at)my-db myhost]# ps -ef | grep pgsql
postgres 19643 5737 0 09:42 ? 00:00:00
/usr/pgsql-9.0/bin/postmaster -p 5432 -D /var/lib/pgsql/9.0/data
postgres 19644 5737 0 09:42 ? 00:00:00
/usr/pgsql-9.0/bin/postmaster -p 5432 -D /var/lib/pgsql/9.0/data
postgres 19657 5737 0 09:42 ? 00:00:00
/usr/pgsql-9.0/bin/postmaster -p 5432 -D /var/lib/pgsql/9.0/data
postgres 19658 5737 0 09:42 ? 00:00:00
/usr/pgsql-9.0/bin/postmaster -p 5432 -D /var/lib/pgsql/9.0/data

pgstartup.log doesn't indicate anything about multiple instances starting,
and indicates that PID 5737 was the initial PID for the instance:

# cat pgstartup.log
2012-09-07 08:01:33 MDT [5737]

Not to mention the fact that a whole series of things would have to go
wrong to have PostgreSQL listening on the same port and different PIDs - so
many I can't even imagine it.

So I'm wondering if this has even been seen before (and/or whether I'm just
chasing windmills here).

Environment:
OS: CentOS 5 - 2.6.18-194.32.1.el5 #1 SMP Wed Jan 5 17:52:25 EST 2011
x86_64 x86_64 x86_64 GNU/Linux
PostgreSQL Version: psql (9.0.4)

Thanks.