How a GTM Audit Uncovered 17 Months of Duplicate Google Ads Conversions

The Problem

Posh Totty Designs, a UK jewellery brand selling online and in retail through Shopify Plus, came to me with two problems stacked on top of each other. GA4 was reporting a heavy chunk of sessions as Unassigned - no source, no medium - meaning a real slice of revenue had no attribution story at all. At the same time, Shopify had announced it was withdrawing support for the store's tracking setup - a GTM custom pixel wired through Shopify's Customer Events framework - on August 26, 2026. Google's own documentation flags this exact implementation as unsupported: "Google cannot guarantee the behavior of Google tags implemented using a custom pixel."

The team had already tried to patch things internally and, in the process, briefly knocked their own Google Ads conversion tracking offline. That's what pushed them to bring in a specialist rather than risk touching the container again, and why the brief was explicit: full access from day one, but no changes to the sensitive Google Ads conversion tag without sign-off first.

What nobody had caught yet was that the Unassigned traffic wasn't the real story. Underneath it was a duplicate conversion tag that had been quietly running since January 2025 - more than a year of Smart Bidding decisions made on a number that was wrong by 3 to 5 times.

The Approach

The brief called for a migration to Shopify's Google & YouTube App, and that was the right endpoint. But before touching anything, I ran a full audit of the live GTM v2.0 container, the theme-level tag loading, the Google Ads account settings, and the GA4 configuration - because a deadline-driven migration is exactly the kind of project where an existing bug gets carried over into the new setup instead of fixed.

That audit surfaced five separate issues, two of them critical enough to be actively corrupting the account's Smart Bidding data before any migration work even started.

Key Changes

1. Identified and removed a duplicate Google Ads conversion tag ("GAds - purchase (OLD)") that had been firing alongside the current purchase tag on every checkout since January 2025 with no order ID for deduplication - inflating the Primary conversion action's reported conversions by 3.3x to 5.4x every month for 17 months, with every Performance Max campaign optimizing on the inflated number.
2. Diagnosed a missing checkout.shopify.com domain in the Conversion Linker tag - the GCLID cookie set on the storefront couldn't cross into the checkout domain, so purchases were recording but Google Ads had no click to attribute them back to.
3. Found a GA4 referral exclusion gap: shopify.com and shop.app weren't on the excluded referrer list, so some checkout and Shop Pay redirects were showing up in GA4 as a separate referral source and stealing credit from the original channel.
4. Corrected two GTM variable bugs quietly degrading Enhanced Conversions match rate - a postal code variable reading the wrong dataLayer key (sending "ENG" or "SCT" instead of an actual postcode) and a currency variable defaulting to EUR instead of GBP.
5. Fixed a "begin_checkout" tag firing on Unlimited instead of Once Per Event, which was inflating the GA4 checkout funnel, and added missing default parameters to two ecommerce tags shipping without currency or value.
6. Migrated all Google tags off the unsupported GTM custom pixel and onto the Google & YouTube App, running Shopify's tag migration wizard to carry over the existing Google Ads purchase conversion cleanly.
7. Ran full QA across storefront, checkout, and order confirmation pages using Tag Assistant and the GA4 Debugger, confirming a single deduplicated conversion per order, correct GBP currency, and working Enhanced Conversions data.

The Results

In the first week after migration, GA4 Unassigned traffic dropped from 41.9% to under 16% - well ahead of the standard monitoring window. That was the visible win. The bigger one was underneath it: the duplicate conversion tag had been inflating Google Ads' Primary conversion action by 3.3x to 5.4x every month since January 2025, which means Performance Max and Smart Bidding had been optimizing against a signal that was wrong by more than 4x on average for 17 straight months.

Google Ads and GA4 tracking now run through a single, Google-supported path - the Google & YouTube App - with a deduplicated conversion count, working GCLID attribution through checkout, and a GA4 channel report that reflects where traffic and revenue is actually coming from. The migration was completed on June 21, 2026, well ahead of Shopify's August 26 deprecation deadline, with a monitoring period built in before the legacy secondary conversion action is removed for good.

Client Review

"Emtiaz delivered an outstanding piece of work. He was engaged to audit our GA4 and GTM setup on Shopify Plus following elevated Unassigned traffic in Google Analytics. What he found went far beyond the original brief. His audit identified a duplicate Google Ads conversion tag that had been active since January 2025, inflating reported conversions by 3.3x to 5.4x every month and rendering our ROAS and CPA figures materially incorrect for over 17 months. He also identified a missing checkout domain in the Conversion Linker, several GTM variable bugs degrading Enhanced Conversions data, and a GA4 referral exclusion issue - all documented clearly with specific fixes for each. He then completed the full migration from GTM to the Google & YouTube App on 21 June, with thorough QA documentation and a clear monitoring plan. The results speak for themselves. Unassigned traffic has dropped from 41.9% to under 16% in the first week post-migration and is still falling. Emtiaz communicated clearly throughout, flagged risks and delivered well beyond the scope of the original brief. I would not hesitate to work with him again and would recommend him without reservation to anyone needing serious GA4 or GTM work on Shopify."

- Posh Totty Designs, UK Jewellery Brand

Engagement: GA4 & GTM Audit and Migration - Shopify Plus | Fixed-price project | Rating: 5.0/5.0

Key Takeaways

• A duplicate conversion tag with no order ID can run undetected for over a year - check every tag hitting a conversion action for deduplication logic, not just the newest one.
• Unassigned traffic in GA4 is a symptom, not a diagnosis - it can come from feed UTM tagging, missing referral exclusions, or a checkout domain gap in Conversion Linker, and each has a different fix.
• A platform deprecation deadline is worth using as a forcing function for a full audit, not just a like-for-like migration - the deadline is rarely the only problem in the account.
• A rising Google Ads conversion count isn't automatically good news - rule out duplication before treating a spike as real performance.
• Enhanced Conversions match rate depends on every upstream variable being correct - one wrong dataLayer key can silently degrade match quality for months without tripping any alert.