Hello everyone and thanks for reading my question. It’s the first one I made on StackOverflow so hopefully you guys can help me out. It might be a bit of a long read, so apologies for that in advance.

I have a client/project that wished to create a new Google Analytics 4 environment that combines data from two seperate domains:

• Their regular website, which I will call domain A
• Their webshop, which I will call domain B

Both Domain A and B were already generating data through their respective GA4 properties. For naming purposes, I will call these GA4 properties GA4_A and GA4_B. Their wish was to keep these properties as they were so their data could be viewed individually, but to also create a property which combines the data of both domains, which I did. I will call this GA4 property GA4_C.

I created a single Google Tag Manager(GTM) workspace in which all the GA4 properties, so A, B and C, are set up. A couple details about the structure are as follows:

• GA4_A will only fire when page_url contains
• GA4_B will only fire when page_url contains
• GA4_C will fire on all pages
• In order to enable cross-domain data collection in GA4_C, I’ve added both “contains ” and “contains ” in the Google Tag settings.

Next, I implemented the GTM workspace in both domain A and B. Here’s a visual overview (which I made in Paint through years and years of experience):

enter image description here

When initially debugging, everything seemed good, no double records, all the set up events only being sent to the GA4 they should be sent to, etc. No problem.

But then later, we’ve started seeing huge increases in traffic for both GA4_A and GA4_B, right around the time these changes were implemented. It’s in Dutch, but I think you will get the gist of it:
GA4_A:
enter image description here
GA4_B:
enter image description here

Perhaps one of you knows what could be the cause? Thanks very much in advance.

First off, I am willing to accept that there’s a big possibility I’m just a complete idiot and I’m missing something really simple here, perhaps I’m overlooking something that’s right in my face. It looks like I’ve made an error somewhere that is causing somewhat duplicate measurements, even though I haven’t found any duplicate events or GA4 initializations using the regular GTM and GA4 Debug modes.

What I did find as a lead, however, is that whenever using the GTM Debug function, my Google Tag Assistant Legacy (Chrome) plug-in is showing duplicate instances of the GTM container as shown below…
enter image description here

But whenever I visit both domain A and domain B and check the same plug-in, it only shows a singular instance of GTM. The GTM Assistant Legacy is also no longer supported, so it might just be untrustworthy data regardless.

I have asked the web developers to manually check the HTML source codes for duplicate GTM instances to rule it out, but no reaction yet.

In the meantime, I wanted to ask you guys if you could perhaps shed some light on the situation and how I can possibly fix it.

Thank you very much in advance. Have a nice day/weekend.

Kind regards,

John