My story is that I was doing some Google Ad Words work already when I decided to try the Google Channel. Eventually, I tried Tag Manager. And very quickly, I ended up with the duplicate conversions error. I have been struggling with this for a long time.
Right now, I have no code installed by me on my site. No UA. No gtag. No Tag Manger. Nothing in Preferences, nothing in Additional Scripts, nothing in my theme.liquid. But my global site tag is firing twice on my product pages.
If I put in my UA for Google Analytics in preferences, Tag Assistant reports that it too is suddenly installed twice.
I believe from my investigations that one or more of my gtags is stored in the dreaded "TREKKIE."
When I chat with Shopify, Assistants say beyond their scope, hire an expert.
I believe a correct install would be Tag Manager in theme.liquid and all conversion actions and Analytics tags in Tag Manager, keeping the Shopify side clean. But this seems impossible for one using the Google Channel, as it seems to create and maintain forever its own Gtag. Maybe more than one if you make enough changes, as I have done.
I found a null command somewhere, but it doesn't solve the problem on checkout, where Tag Assistant still shows a bazillion duplicate tags firing on the Thank You Page.
Does anyone have a fix for this? Dire straits here. Thank you in advance to anyone who takes precious time to help out this stranger.
Did you find a solution to your issue? I have a similar issue, suspected this Trekkie thing as well, got a similar answer from support.
We did not find a solution for a DIY installation of tag manager that would not trigger the double-conversions warning. Calculating whether conversions were really being doubled was... complicated and unresolved. The step we took in the interim was to install one of the highly rated feed apps that manages its own tags.