rec: when merging two YAML vectors, check for duplicates

[omoerbeek]

See the issue described in the merge docs (pdns/recursordist/settings/docs-new-preamble-in.rst).

[omoerbeek]

This cannot be done in general. We even shows an example where a duplicate value in a vector can be of use in https://docs.powerdns.com/recursor/settings.html#dont-throttle-netmasks (and the corresponding YAML version)