When making changes through the DNS Manager, you can select whether to apply these changes to a single domain, or to all domains parked to the account. You can do this through the "Apply changes for:" select box.
If you choose "All domains in this account", the changes you make through the interface will be applied in the DNS configuration of the main and all domain names parked to the account.
When adding a DNS record for "All domains in this account", this DNS record will be added for all parked domains, no matter if a record of the same type exists for the same hostname (subdomain). If the target of the record is different from the existing record, then it is possible the particular hostname to receive two records of the same type. For example, you have an existing A record for test.your_parked.domain.com pointing to 18.104.22.168. If you add another A record for the test subdomain for "All domains in this account", pointing to 22.214.171.124, then test.your_parked.domain.com will end up with two A records - one pointing to 126.96.36.199, and another pointing to 188.8.131.52.
Mail Delivery status
When applying changes to the mail delivery option for "All domains in this account", the existing mail delivery options of the main and all parked domains will be overwritten with the one you are currently setting.