A SOC analyst reviewing switch logs and host ARP caches notices that two different IP addresses on the same VLAN — the default gateway (192.168.10.1) and a finance workstation (192.168.10.55) — are both mapped to the same MAC address (00:1A:2B:3C:4D:5E), which belongs to an unknown host. Users on that VLAN report intermittent slowness, and packet captures show duplicated traffic. Which type of network attack does this pattern most strongly indicate?