Sure, I can help you with that. Although the Google Tag Manager (GTM) is one common cause of this issue, it could potentially be caused by several other factors as well. It’s possible that there may be some sort of caching that’s causing the event to fire twice, especially if you’re using a caching plugin on your site. I would try clearing any caching plugins that you’re using and see if the issue persists after that. You also could be having an issue with the theme or another plugin causing a conflict. To isolate this, you could try disabling other plugins briefly to see if the double ‘begin_checkout’ events stop firing.
It’s also important to take into consideration that the way in which Google Analytics events are set up may cause such an issue. For instance, they might be triggered both when the checkout button visibility changes, and when it is clicked. This may require having a developer take a look at the JavaScript related to the event triggering.
I would recommend sharing these possible scenarios with your client for them to investigate further and potentially consult with a developer if needed. If nothing else works, it’s also possible to use the Measurement Protocol API to manually remove duplicates, although this would be a more technical and complex approach.