Why are there multiple devices with the same name listed in the Client List?

A MAC address is a unique network address for a device to identify itself to the network. When a device with an unknown MAC address joins the network, the network will identify it as a new device and then list it in the Client List.

To further protect user privacy, iOS 14, Android 10, and later versions have released a new feature called Private Address. Devices of these versions can use a different MAC address with each Wi-Fi network, and this unique, static MAC address is the device's private Wi-Fi address for that network only.

Since Private Address is enabled by default, each time the device joins different networks (including different bands), it will be identified as a new device and listed in the Client List with a different MAC address.

It is safer for a device to connect to a public WiFi using a Private Address. However, in-home networks, some features like Parental Controls, QoS, and Blocked Devices are implemented based on MAC address, so if a device is using a private address, don’t forget to add it to the controlled list. Or you can also disable the Private Address feature on your clients following the instructions:

iOS14:  Setting -> WLAN -> tap on the blue info icon at the left of the Mercusys router’s Wi-Fi name -> disable the Private Address feature. 

(Picture refer to https://support.apple.com/en-us/HT211227)

Android: Settings app -> Network and Internet -> WiFi ->Connect to Mercusys router’s wifi -> Tap the gear icon next to the current Wi-Fi connection -> Advanced ->  Privacy -> "Use device MAC"

Window 10: Setting -> Network and Internet -> "WiFi" tab -> "Advanced Options." -> "Off" under "Random Hardware Address."  -> Restart your computer. 

(Picture refer to https://it.nmu.edu/docs/disable-random-hardware-address-windows-10