If Microsoft Office Communications Server 2007 R2, Group Chat server is deployed in a Windows Server 2008 R2 domain, you cannot sign in with Group Chat client or with Group Chat Administration Tool. Additionally, you receive the following error message on the Group Chat server:
SIP_E_AUTH_UNAUTHORIZED Message="An item with the same key has already been added."
This issue occurs because the Chat client and the Group instructed Unified Communications Client (UCC) API to only use NTLM for authentication. NTLM sign in fails and UCC API should fail over to Kerberos authentication in a Windows Server 2008 R2 domain.
To resolve this issue, apply the following update package
982128 Description of the update for the Office Communications Server 2007 R2 Group Chat Administration Tool: April 2010
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.