Two Macs signed into Google with same IP address
If you recently updated Google Chrome using the built-in updater (available from chrome://help/), that could cause Google to recognize two different browsers.  If you haven't updated Chrome, or just want to go through the process:

Click on the Google Account icon in the upper right-hand corner of your Gmail window and click on the My Account link.

From there, choose Sign-In &amp; Security, scroll down to Recently Used Devices and click on Review devices (aka the Devices &amp; Activity page as described in Google's Security support article).

Click on Secure Your Account and follow the various prompts; you'll be signed out of your Google account and required to re-enter your password or possibly authenticate via 2FA, if you already had that enabled.

After you have logged back in, return to the Recently Used Devices to verify only one browser.