This article provides best practice information for assigning static MAC address to a Virtual machine vNIC and adding a new vNIC to the Virtual Machine.

VMware vSphere ESXi 6.7

Before assigning the static MAC address, refer the table MAC_ADDRESS table(select * from VPX_NIC) in the VCDB to make sure the there is no duplicate.(Irrespective of upper or lowercase)

Use lowercase MAC address and do not use uppercase letters. vCenter server will sometime fail to detect the Uppercase Lowercase duplication.

Refer the VMware document for more details.
https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.networking.doc/GUID-F9243FED-F081-498F-B4A9-EF950292AF77.html

When assigning the vNIC to the Virtual machine, assign it either from vCenter web client or Host client. Refrain from assigning/connecting the vNIC from VMRC console or any other methods.

For any scenarios of MAC address duplication, Change the MAC address of a virtual machine network adapter manually.

Reference : https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.networking.doc/GUID-8D7D0126-8E8A-470F-A61E-4197EE32D08F.html

https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.networking.doc/GUID-F9243FED-F081-498F-B4A9-EF950292AF77.html

https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.networking.doc/GUID-8D7D0126-8E8A-470F-A61E-4197EE32D08F.html