In your DNS settings, you can use the same name for records of different types. For example, you can create an A record with the name ‘subdomain’ and an MX record with the name ‘subdomain’. Both records will then apply to subdomain.yourdomainname.nl.
The exception to this is a CNAME record. For a CNAME record, the name you use must always be unique. You cannot have any other record with the same name. For instance, if you have a CNAME record named 'subdomain', no other record is allowed to use the name ‘subdomain’.
For the same reason, it is also not possible to set a CNAME on the record with the name @. The @ record always contains an additional SOA record behind the scenes, which means the name would no longer be unique.
If you receive the error message ‘A CNAME record must have a unique name’, the best thing to do is check the names of your DNS records and make sure that the name of your CNAME record is not also used by another record.
More information about the DNS RFC can be found on the following page: http://www.ietf.org/rfc/rfc1035.txt