Problem Parking Domain

I have an existing account on my server, domain.com. It has email accounts associated with it. I want to now park that domain on top of another domain but get the below error: There was a problem parking (aliasing) the domain "domain.com" on top of the domain "otherdomain.com": (XID 8k782r) A DNS entry for "domains.com" already exists. You must remove this DNS entry from all servers in the DNS cluster to proceed. I do not want to terminate domain.com and start over because it will delete all the email accounts and email associated with it. How do I park domain.com on top of otherdomain.com while keeping the existing email accounts intact?