Right 
But there are others that haven't queried pihole today and haven't got a duplicated entry.
Why are you using the arp cache as comparison instead of pihole's network table? This would circumvent the problem with clients not seen for a while (no info in the ARP cache) because they are still in the network overview? I guess there is a drawback I don't see right now... 
This reminds me that I've seen a feature request somewhere to be able to delete single entries from the network table via web UI 