"Cannot insert duplicate key row in object 'dbo.Agents'" error when adding an RGS agent to a group via an Exchange distribution list

Consider the following scenario:

• You have a user forest and a resource forest in your Active Directory environment. The Microsoft Exchange Server and Skype for Business Server are hosted in the resource forest; Active Directory users are hosted in the user forest.

• You create a user account in the user forest, for example, user U. The objectSID is objectSIDU.

• You create a disabled account in the resource forest, for example, user R, and populate the msRTCSIP-OriginatorSID with objectSIDU.

• You enable the user R in Skype for Business Server 2015 with Sip address: sip:user@contoso.com.

• On the Exchange Server, you create a mailbox for user R with the email address: smtp:user@contoso.com and create a group, group@contoso.com, then add the user R as a member of this group.  

• On the Skype for Business Server 2015 Control Panel, select Response Groups, click Group, then click New to create an RGS group that uses name Group A and add the agent user R.

• You create another RGS group that uses name Group B, select Use an existing email distribution list under Agents. Input the distribution list address group@contoso.com that's created before.

In this situation, you may receive an error message that resembles the following:

This issue occurs because there's an existing entry in the agent table that's used by the response groups. The dbo.agent table will be populated with a different SID for user R (the disabled account that you created in the resource forest). Therefore, it causes the duplication.

To fix this issue, install the July 2018 cumulative update 6.0.9319.534 for Skype for Business Server 2015, Front End Server and Edge Server.