When a GUID is not really unique - I'm looking at you SharePoint!
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Given (recent) changes and a new feature called Fast Site Collection Creation, used in SharePoint Online and to some extent in SharePoint 2016, this is no longer true and you can no longer trust in the Web Id being unique. When a site collection is created with a root web, specifically the SharePoint Team Site (STS#0), it is copied from a site master. The Site Collection is given a new GUID, but the GUID for the root web is copied from the site master. So, in SharePoint Online all your Site Collections with a root site of type Team Site will have the same id.
There is no workaround for this, you cannot change the Id - your only chance is to use a combination of Site Collection Id and the Web Id. That is a dual GUID combo - can't be more unique than that.
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Wictor Wilén - SharePoint MCA, MCSM, MCM and MVP - When a GUID is not really unique - I'm looking at you SharePoint!
http://www.wictorwilen.se/when-a-guid-is-not-really-unique-i-m-looking-at-you-sharepoint