When you use unicast NLB, a virtual media access control (MAC) address is generated automatically based on the cluster IP address. When you run in multicast mode, a multicast MAC address is used. The address conflict occurs because the multicast MAC address and the unicast MAC address do not match. See ME282023 to solve this problem.
As per Microsoft: "Media access control addresses are preassigned and permanently burned into the network interface card (NIC). These addresses under normal circumstances are always unique, however, rare errors made during the manufacturing process can cause duplicate media access control addresses to be used and cause this problem.". See ME164903.