Okay, do you own these devices? At some point they were registered with your Amazon account. And you have control over them with your Alexa app. If you are the Amazon account owner. This is what I want you to do. Ready? From your mobile device access your Amazon app and login in. (The following can done from a browser. Although, you’ll access - Your Account to the right of the search bar).
Bring up the menu (the three lines on the top left).
Click, Your Account > Manage content and devices > Devices (next to Content at the top).
From there you will see all your Amazon devices. Select the FireTV you want to remove and deregister it. The serial numbers will be listed. If you can’t tell by the date registered which one you want to remove - you can find the serial number for the Fire TV you want to keep from the Setttings menu on your Fire TV - Home screen.
To expand on my above post. You can also see the details of your Fire TV devices from your Alexa app.
On the bottom of your screen, click **More ** > **Settings ** > **Device Settings ** > From there you can click on the Fire TV stick device and at the bottom click, About. You could cross reference the serial numbers on both Fire TV sticks. If the serial numbers are the same but the Mac address are different, try rebooting your Wifi router. And check again.
Hi neighbors. I do not see an option allowing me to do this in the Alexa app. I would recommend double-checking with Alexa support to see if this is possible.