@MaxineN I tested this a few days ago when I first saw the thread. I've got a Unifi Cloud Gateway, on 1000/500 in Auckland with three Unifi APs serving ~40 wifi clients. Wifi being wifi and quiet lossy, and some clients being out in the garden shed I do have a quiet a few retries reported in the Unifi dashboard, so it's somewhat unsurprising to see the duplicate acks on the WAN interface.

I've not noticed any real-world issues, but it has prompted me to look into my new wifi setup... Looks like I need a dedicated SSID for IoT devices and to turn down my power, I'm seeing up to 25% retries on my outside AP and 10% on my two indoor ones. 

If I could bump everyone off the wifi i'd do a test with one device on the lan to see if I get any/many duplicate acks, but that'll have to be planned to avoid a riot at home :D