nrpe.spec %pre does

# check if the port for nrpe is already defined in /etc/services
if grep -q %{nrpeport} %{_sysconfdir}/services ; then
    : OK - port already defined
else
    %{nnmmsg} "Adding port %{nrpeport} to %{_sysconfdir}/services"
    echo "nrpe            %{nrpeport}/tcp # Nagios nrpe" >> etc/services
fi

With usrmerge and /usr/etc/services, this means that installing nrpe will
create /etc/services with just one line:

    nrpe            5666/tcp # Nagios nrpe

Needless to say that you can find exactly this line in /usr/etc/services ;-)

Please adjust the %pre script to also check /usr/etc/services.