Eliminating duplicate Google Ads conversions with GTM

A conversion counted twice isn't just a dirty number. It carries through into everything you decide next.

• Your data stops being true, and your team believes it's performing better than it is.
• You allocate budget on inflated figures, putting money into channels that haven't earned it.
• You pay for advertising based on conversion data that's wrong. That costs money directly.

• Visitor behaviour: someone refreshes the thank-you page or returns to it later, and the tag fires again.
• Technical hiccups: a slow connection or browser issue leads people to resubmit a form or order.
• Tagging mistakes: duplicate or redundant tags writing the same event twice.

In GTM you can shut this down with Simo Ahava's Transaction ID logger. It stores the transaction ID in a cookie or localStorage under the key __tid_gtm.

On the next purchase event you compare the transaction ID against what's in that cookie. If they match, you block the tag. The duplicate is stopped before it exists.

• Get the tag template from the Community Template Gallery.
• Attach it to your purchase trigger so the transaction ID lands in the cookie.
• Create a first-party cookie variable with the value __tid_gtm.
• Feed that cookie variable into a regex table variable.
• Add a row with the transaction ID as the pattern and true as the output.
• Create a custom event trigger using regex .* so it matches any event name.
• Let that trigger fire only when the regex table returns true.
• Finally, use it as a blocking trigger on your purchase events.

This intervenes in your purchase measurement, so test the whole chain before rolling it out widely. One misplaced blocking trigger and you measure no conversions at all.