Consider next UserParameters entries in agentd config file:

UserParameter=custom.echo[*],echo $1
UserParameter=custom.echo[*],echo $1
UserParameter=custom.echo[*],echo $1
UserParameter=custom.pgsql.version,psql --version | head -1
UserParameter=custom.pgsql.version,psql --version | head -1
UserParameter=custom.pgsql.version,psql --version | head -1

Let's print available commands:

$ sbin/zabbix_agentd -p | grep custom
custom.echo[] [m|ZBX_NOTSUPPORTED]
custom.echo[] [m|ZBX_NOTSUPPORTED]
custom.echo[] [m|ZBX_NOTSUPPORTED]
custom.pgsql.version [t|psql (PostgreSQL) 8.4.7]

Duplicate Flexible UserParameters are all added as is, Simple UserParameters are replaced. We agreed that for any UserParameter in case of duplicate key an error should be reported and agent exited instantly.