你无法创建一个相同的用户并mapping这个用户到数据库， 并且你无法删除数据库的用户：DemoUser.
请运行以下脚本， 之后你就可以删除用户：DemoUser.然后创建用户（DemoUser.）并mapping到数据库。
脚本：
GO
ALTER AUTHORIZATION ON ROLE::[aspnet_Membership_BasicAccess] TO [dbo]
GO
GO
ALTER AUTHORIZATION ON ROLE::[aspnet_Membership_FullAccess] TO [dbo]
GO
GO
ALTER AUTHORIZATION ON ROLE::[aspnet_Membership_ReportingAccess] TO [dbo]
GO
GO
ALTER AUTHORIZATION ON ROLE::[aspnet_Profile_BasicAccess] TO [dbo]
GO
GO
ALTER AUTHORIZATION ON ROLE::[aspnet_Profile_FullAccess] TO [dbo]
GO
GO
ALTER AUTHORIZATION ON ROLE::[aspnet_Profile_ReportingAccess] TO [dbo]
GO
GO
ALTER AUTHORIZATION ON ROLE::[aspnet_Roles_BasicAccess] TO [dbo]
GO
GO
ALTER AUTHORIZATION ON ROLE::[aspnet_Roles_FullAccess] TO [dbo]
GO
GO
ALTER AUTHORIZATION ON ROLE::[aspnet_Roles_ReportingAccess] TO [dbo]
GO
GO
ALTER AUTHORIZATION ON SCHEMA::[db_owner] TO [dbo]
GO
GO
ALTER AUTHORIZATION ON SCHEMA::[db_owner] TO [db_owner]
GO