In class CL_CRM_BUPA_SERVICE and method PREPARE_CPWORK_SEARCHFIELDS, add field ADDRNUMBER to adrc_flds. This is an SAP code modification so you need to register for an access key.
Figure 1. Add ADDRNUMBER to adrc_flds 
Note: Contact Person duplicate checking may be limited in some releases of the CRM
WebClient. This is particularly evident in the Customer Interaction Center Web Client.
For example, the duplicate check is not called by SAP when using the Create Related
Contact Person function of the Identify Account page in the Interaction Center WebClient
in CRM 2007.