Why it happens
The pixel and the Events API both send the purchase, without a shared event_id. TikTok's help says deduplication "is required if you use both the TikTok Pixel and Events API" for the same event, and that you "must share Event ID (event_id) as a parameter through both Pixel and Events API". If the browser copy and the server copy carry different IDs, or none, TikTok keeps both.
Two apps or tags send purchases. The TikTok sales channel sends events from your store. A TikTok pixel pasted into the theme, added through Google Tag Manager or installed by another app sends its own. Each fires on the same order. TikTok's deduplication only joins copies that share the same event name and event_id, and two different tools rarely share an ID.
The thank you page fires again. A buyer who reloads the order status page can fire the pixel a second time. TikTok's help says it deduplicates pixel events with an identical event and event_id within 48 hours of the first. If the second fire carries a new ID, it counts as a new purchase. Purchase fires again when the order page is reopened covers this for Meta and GA4; the same cause applies.
A merge needs time. For a pixel event and an Events API event, TikTok says it merges copies with the same event and event_id "arriving after 5 minutes and within a 48-hour window from the first event". A copy sent days later, for example by a tool that retries old orders, falls outside the window and counts again.
Two event names for one sale. TikTok renamed Complete Payment to Purchase and maps the old name to the new one, so a store that sends either is fine. A store that sends a Purchase and also a Place an Order event for the same sale has two events. Only one should be the event your ads optimise for.
TikTok counts in its own way. Not every extra purchase is a duplicate. TikTok's help says its pixel is multi-session: it credits a purchase when a person clicks an ad and comes back later to buy within the window. Google Analytics may credit the same sale to search or e-mail. Shopify counts the order once. Both can be right. A duplicate shows up when TikTok counts more purchases than you had orders at all.
Check it yourself
- 1In Shopify admin > Orders, count paid online orders for the last 7 full days. If TikTok's purchase count for the same days is higher than your total orders, duplicates are almost certain.
- 2In TikTok Events Manager, open your pixel's Overview tab and find Purchase (or Complete Payment). Read its connection method. TikTok shows whether an event comes "browser only", "server only", or "server & browser".
- 3If it says server & browser, open the event details. TikTok shows deduplication information there.
- 4Open Test events and place a test order. Count how many Purchase events arrive and from where. One from the browser and one from the server, with the same event_id, is fine. Two from the browser, or two with different IDs, is the problem.
- 5List every place that can send a TikTok purchase: the TikTok sales channel and its data sharing level, your theme code, Google Tag Manager, Customer events in Shopify settings, and other apps with a TikTok setting.
- 6Open the Diagnostics tab. TikTok lists issues with their severity, and the ads they affect.
- 7Check which event your ad groups optimise for, and that only one purchase event is used.
If your TikTok count is lower than Shopify, not higher, read TikTok shows fewer purchases than Shopify instead.
FAQ
Does TikTok remove duplicates by itself?
Only when the copies share the same event name and event_id within 48 hours. Copies with different IDs both count.
Do I need deduplication if only the server sends purchases?
No. TikTok's help says deduplication is not needed when the pixel and the Events API send different events, for example add to cart from the browser and purchases from the server.
Will duplicates hurt my campaigns?
They can. TikTok learns from the purchases it receives. If each sale counts twice, your cost per purchase looks half as high as it is, and budget decisions follow a number that is too good.
Which copy does TikTok keep?
TikTok says that when it recognises copies of the same conversion, it keeps the first event received and enriches it with the other.
Why does TikTok show more sales than Google Analytics?
Partly by design. TikTok credits sales when a person returns later after a click, and its in-app browser can hide the referral from Google Analytics. Compare TikTok with your Shopify orders, not with GA4, to find duplicates.