User Mapping question

SS05/SSMS

I select Login Properties for a domain account and select User Mapping.

I'd like to map this login to the dbo user on a couple of dbs. However, if
I check the "Map" checkbox and enter "dbo" for the User column and then
click "OK" I receive the error :

"Create failed for User 'dbo' (Microsoft.SqlServer.Smo)
Additional information:
An exception occurred while executing a Transact-SQL statement or batch.
(Microsoft.SqlServer.ConnectionInfo)
User, group, or role 'dbo' already exists in the current database.
(Microsoft SQL Server, Error: 15023)

So apparently SSMS attempts to create the specified user in the specified
databases -- when I wanted a mapping.


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