According to the ServiceNow Discovery documentation, the base system CI Identifiers for Windows Server class are name and serial_number. These are the attributes that Discovery uses to uniquely identify a Windows Server CI. If both attributes match an existing CI, Discovery updates that CI. If only one attribute matches, Discovery creates a new CI. If neither attribute matches, Discovery also creates a new CI. In this scenario, the serial_number is the same for both Windows Servers, but the name is different. Therefore, Discovery will create two separate CIs, one with WindowsSN1 as the name and one with WindowsSN2 as the name.

References:

Create a Discovery CI classification - Product Documentation: Tokyo - Now Support Portal

Identification rules - Product Documentation: Tokyo - Now Support Portal

ServiceNow IRE: Identification Rules Explained — Cookdown