arp list shows same ip and mac address on both wan and lan

I have dynamic arp enabled on both wan and lan. LAN interface is configured with multiple IP addresses on different subnets. WAN interface is configured with public IP from a /29 subnet. It seems that randomly, IP addresses from subnets configured in LAN and WAN appear in both interfaces simultaneously and after a few sec they are removed again. Any explanation? Thank you, imitrushi