A missing dedup key doubled every TikTok purchase — the real CPA was 2.1x

The situation

An Australian activewear brand had correctly implemented both the TikTok pixel AND the Events API (rare). Their TikTok rep praised their setup. But conversion volume was 2x what Shopify showed and media buyers couldn't reconcile the numbers.

The problem

The TikTok pixel was firing on the confirmation page. The Events API was sending a purchase event from a Shopify webhook. Both were correctly implemented. The missing piece: no event_id was set on either side. TikTok's deduplication algorithm requires a matching event_id on both the pixel event and the CAPI event to identify them as the same purchase. Without it, TikTok counts both as separate conversions.

What we did

1. Audited TikTok pixel network calls — event_id field was absent from the pixel payload
2. Audited Events API payload in TikTok server logs — event_id field was also absent
3. Added event_id = "purchase_" + Shopify order ID to the pixel event via GTM dataLayer
4. Added the same event_id format to the Shopify webhook payload sent to TikTok Events API endpoint
5. Confirmed dedup working within 24 hours — conversion volume halved to match Shopify

Why it worked

TikTok's documentation recommends event_id for deduplication but doesn't make it mandatory — the Events API accepts events without it. This means a correctly-deployed CAPI + pixel stack can still double-count every conversion if event_id is absent. TikTok won't warn you. The only way to catch it is comparing CAPI conversion count against Shopify order count.