Consider the following scenario:
You sign in to Outlook Web App (OWA) and then open Exchange Control Panel (ECP) in a Microsoft Exchange Server 2010 environment.
You open a large distribution group that contains more than 1,800 recipients, and then you expand the Membership tab.
You add or remove a recipient in the members list, and then click Save.
In this scenario, you receive the following error message:
The recipient "MailboxName" is already a member of the group "GroupName".
This issue occurs because of a logic defect in the ECP code. Specifically, ECP calls only the Update-DistributionGroupMember cmdlet instead of calling the Add-DistributionGroupMember or Remove-DistributionGroupMember cmdlet for large distribution groups.
To resolve this issue, install the following update rollup:
2866475 Description of Update Rollup 2 for Exchange Server 2010 SP3
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.