I actually currently have this issue, but have chosen to ignore it since it hasn't had any impact on anything. What'd I'd do is look up the MACs by going to Network Wide -> Clients and see what comes up. Maybe there are 2 devices with same MAC address but with different IPs? Maybe 2 of the same IPs on 2 different MACs? See where they're getting an IP from. You can also check to make sure they are in their proper VLAN. But, this happened on my MX and I believe it's because of the way I configured the ports on it. However as I mentioned, it hasn't impacted anything so I let it be.