I had the same issue when attemping to extend our Schema for OCS. I had to open a ticket with MS PSS. I followed the steps in the Exchange KB but that only added a step to the resolutoin. The resolution is listed below:
PROBLEM
========
Trying to extend the schema for OCS 2007. They had previously extended the schema with Aladdin etoken, which is used for smart card "etoken" certificates. The link ids appear to be the same. We get the error "schema update failed An attribute with the same link identifier already exists." After this, article 841670 “Schema update failed” error message when you run the setup /forestprep command or you run ADC Setup in Exchange 2000 Server or in Exchange Server 2003 was used to try to get a couple of the attributes into the schema. Only msRTCSIP-MCUFactoryAddress with linkid 11900 actually made it to the schema.
RESOLUTION
===========
A KB article is pending on this issue. The article number will be 942758 Error message when you extend the Active Directory schema for Office Communications Server 2007: "Extending the schema in Active Directory failed".
With the assistance of Tarun from Directory Services support, we changed the link ID for msRTCSIP-MCUFactoryAddress from 11900 to 14946 using LDP.
We copied over the OCS files from the CD to the hard drive.
We modified the schema.ldf file, in the same directory as OCS setup.exe, with the following linkid changes:
From:
11024 - msRTCSIP-MCUFactoryPath
11025 - msRTCSIP-SIP-PoolAddresses
11026 - msRTCSIP-MCUFactoryAddress
11027 - msRTCSIP-MCUServers
To:
14944 - msRTCSIP-MCUFactoryPath
14945 - msRTCSIP-SIP-PoolAddresses
14946 - msRTCSIP-MCUFactoryAddress
14947 - msRTCSIP-MCUServers
Then we ran Setup... Forest Prep which succeeded without error.
