It may seem weird, but yes - SharePoint Web GUID is not unique across the tenant/farm. And it has never been.
It is unique only inside a site collection.
It can be confusing (everyone knows what acronym means), most of us probably has never noticed that... But it is what it is.
And you'll probably notice it more often with fast site creation for modern Team and Comm sites in SharePoint Online.
So remember that fact... and modify your code if it based on assumption that the Guid is unique.
Few more words by Wictor Wilen: http://www.wictorwilen.se/when-a-guid-is-not-really-unique-i-m-looking-at-you-sharepoint

That's all for today!
Have fun!