I would venture that, since the Rokus are working correctly, then you need to do nothing to "fix" the problem as the problem does not really exist. If it ain't broke don't fix it!
You could try a factory reset on the non-identifying Roku but that does not seem to be needed as it is working.
The reason I wanted to fix it is because I want to verify that the unknown device is actually the roku and not my network compromised. Its just annoying me.
How can you tell that you have an unknown device in your network? Did you check the DHCP client table in your router GUI?
If so, the Roku player is usually labelled as NP-(serial number of your Roku player).
I checked my router and enabled MAC filtering and noticed it. I always knew this information would come in handy.
Or if you want to get down and dirty, you can use a Windows based computer and check the IP of all the devices connected to your network.
Just open the command prompt and enter the command "apr -a". It will then show the MAC address off all the devices in the network along with their respective IP address.
This is a bit tedious, so I would suggest that you sign in your router GUI.