Meta Conversions API deduplication not working? How to fix event_id on Shopify

Sending purchases through both the Meta pixel and the Conversions API is the setup Meta recommends. The browser catches the session details, the server catches the purchases the browser misses. But it only works if Meta can tell that the two events describe the same order. When it can't, every purchase gets counted twice.

That matching is called deduplication, and it runs on one field: the event ID. This guide explains exactly how Meta uses it, the six ways it goes wrong on Shopify, and how to prove with one test order that it's fixed. If you already know Meta shows more purchases than Shopify, start with Meta duplicate purchases on Shopify, which covers every cause, then come back here for the event ID side.

How Meta deduplicates events

Meta's documented rule has three parts. A browser event and a server event count as one when:

1. The event names match. Purchase in the pixel and Purchase in the Conversions API.
2. The event IDs match. In the pixel, the ID is eventID, passed as the fourth argument of fbq('track', ...). In the Conversions API, it is event_id.
3. They arrive within 48 hours. Meta only deduplicates events received within 48 hours of the first event it got with that ID.

A purchase sent correctly from both sides looks like this:

// Browser (pixel)
fbq('track', 'Purchase', { value: 84.00, currency: 'USD' }, { eventID: '5739204817' });

// Server (Conversions API)
{
  "event_name": "Purchase",
  "event_id": "5739204817",
  "event_time": 1762330000,
  "action_source": "website",
  ...
}

When both match, Meta generally keeps whichever arrived first and discards the other. There is a second, weaker method based on the fbp cookie or external_id, but it generally only works when the browser event arrives before the server one, and it can't catch duplicates from a single source. For purchases, use event IDs.

Check whether it is working

1. Compare counts first. In Events Manager, open your dataset and note Purchase events for the last 7 days. Compare with Shopify online store orders for the same days. If Meta's number is more than about 5% higher, something is not being deduplicated.
2. Open the Purchase event. Click the Purchase row in Overview. You should see both Browser and Server as connection methods, and the event details show how Meta is deduplicating them and whether event IDs are being received.
3. Read Diagnostics. Warnings about duplicate events or missing deduplication keys tell you which event is affected. Fix Purchase first, then the checkout steps, because those are what your campaigns optimize on.

The 6 reasons it fails

1. Each side makes up its own ID

The most common cause. The browser generates a random ID when the page loads, the server generates a different one when the order webhook arrives, and they never match. Both events are counted.

2. Browser and server come from different tools

For example, the Facebook and Instagram app sends the pixel, and a tracking app or server-side GTM sends the Conversions API. Each tool uses its own ID scheme. Unless you've configured both to use the same value, they won't match. The usual fix is to let one tool own both sides.

3. The ID is missing on one side

Older theme code calls fbq('track', 'Purchase', ...) with no fourth argument, so the browser event has no ID at all. Or a server integration leaves event_id empty. An event without an ID can't be matched to anything.

4. The event names differ

Purchase in the browser and a custom name like purchase_complete on the server are different events to Meta, even with the same ID. Use the standard event names on both sides.

5. The server event arrives too late

Events more than 48 hours apart are not deduplicated. This happens with batch uploads, delayed webhooks retried days later, or orders that are only sent once they're paid or fulfilled.

6. The ID changes when the page reloads

If the purchase fires on the order status page and the ID is generated on each load, a customer who returns to check their order sends a new purchase with a new ID. Fire purchases on Shopify's checkout_completed event, which happens once per order, and use a stable ID.

Getting event IDs right on Shopify

The rule is to use a value that both the browser and the server already know for the same action. For each Shopify event, that usually means:

In practice, most stores don't write this themselves. There are three clean options:

• The Facebook and Instagram app alone, at its highest data sharing level. It sends both the pixel and the Conversions API and handles the IDs. Nothing else should send to the same pixel.
• A tracking app such as Elevar or Littledata, which owns both sides. The Facebook app's own data sharing then has to be turned down, as the app's setup guide describes.
• Server-side GTM, where you build both tags and pass the same ID to each. Most control, most work. See Stape vs Elevar for how the options compare.

Mixing them is what breaks deduplication. Pick one owner for Meta events and remove the rest.

Test it with one order

1. Open Test events. In Events Manager, go to your dataset, Test events. If your server setup supports a test event code, add it so server events show up there too.
2. Place an order. Use a 100% discount code, or a cheap product you refund afterwards.
3. Check what arrives. You want exactly one browser Purchase and one server Purchase, with the same event ID, and the server one marked as deduplicated. Two browser purchases, two server purchases or two different IDs mean the problem is still there.
4. Reload the order status page. Nothing new should arrive. If a second purchase does, it's firing on page view.
5. Watch the ratio for a week. Meta purchases divided by Shopify online orders should settle near 1.0.

Once the counts are right, the next thing to improve is how well Meta can match those purchases to people, which is what Event Match Quality measures. Expect your reported ROAS to drop when duplicates disappear. Your real results haven't changed, you're just seeing them accurately, which is the point. Our guide to why Meta revenue doesn't match Shopify explains the gap that remains.

Common questions

Owais Alam

Freelance data and analytics engineer and founder of Good Numbers. Top Rated on Upwork. Specializes in GA4, Google Tag Manager, BigQuery, server-side tracking and Shopify attribution.

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