I have a Joomla website with existing analytics done by a GTM tag and I need to add another image tag (1x1 pixel) from a separate GTM container. I set the event name for the additional GTM container to a different name from from the existing GTM container (default 'gtm.js') so that the new tag only fires under the specified event. The new container tested out fine in a blank HTML page, but when it is put in the website, it ends up firing twice. I know that because Firebug showed 2 1x1 pixels being loaded and I mark each request with a randomly-generated UID to distinguish them on the server side.
I suspect this being caused by having multiple GTM container tags but want to check whether anyone has run into this problem before? As of now, I could not verify nor fix the double counting problem.