Why is my Meta Pixel and CAPI double counting purchases?

The two most common causes are a missing event_id on the Pixel event, and an event_name case mismatch — for example 'Purchase' from one source and 'PURCHASE' from the other. Either one stops deduplication and the sale is counted twice.

How do I fix Pixel + CAPI double counting?

Generate the event_id once, client-side, at the moment of conversion, and send that exact same value with both the Pixel and the CAPI event. Keep the event_name identical and case-consistent. Then verify in Events Manager Test Events that both events share the id.

Meta Pixel + CAPI Double Counting: the event_id Deduplication Diagnostic

Q: How does Meta deduplicate Pixel and CAPI events?

Meta treats a browser Pixel event and a Conversions API event as the same event when they share an identical event_id (character-for-character) and an identical, case-sensitive event_name, within a roughly 48-hour window. If both match, Meta counts the purchase once.

If Meta’s Ads Manager reports more purchases than Shopify actually had, your Pixel and Conversions API (CAPI) aren’t deduplicating — Meta is counting each sale twice, inflating your conversions and ROAS. (This is the Meta-side twin of duplicate GA4 events.)

Deduplication isn’t automatic magic — it depends on two fields matching exactly. Here’s how it works and the two mistakes that break it.

How deduplication actually works

When you send a purchase through both the browser Pixel and the server-side CAPI (which you should — it’s how you get high Event Match Quality), Meta needs to know they’re the same sale. It deduplicates when all of these hold:

Requirement	Rule
`event_id`	Identical, character-for-character, on both events
`event_name`	Identical and case-sensitive (`Purchase` ≠ `PURCHASE`)
Timing	Both arrive within the ~48-hour deduplication window

Match all three → counted once. Miss any → counted twice.

The two mistakes that break it

Missing event_id on the Pixel. The most common cause. The CAPI event has an id, the Pixel event doesn’t, so Meta can’t pair them.
event_name case mismatch. One source sends Purchase, the other PURCHASE (or a custom variant). Case-sensitive means these are treated as different events.

A third, subtler cause: generating two different event_ids (e.g. one on the client, a new one on the server) instead of sharing one.

The fix

Generate the event_id once, client-side, at the moment of conversion (e.g. a UUID on the thank-you page).
Send that exact value with both the Pixel event and the CAPI event.
Keep event_name identical and case-consistent across both.
Make sure both fire within the 48-hour window (retries beyond it get re-counted — use idempotency).

How to verify

In Events Manager → your dataset → Test Events, place a test order and watch the Purchase event. You should see it flagged as received from both Browser and Server, deduplicated into one — and both carrying the same event_id. If you see two separate purchases, re-check the id and the event name casing.

FAQ

How does Meta deduplicate Pixel and CAPI events? Same event_id (character-for-character) + identical case-sensitive event_name, within ~48 hours → counted once.

Why is my Pixel and CAPI double counting? Usually a missing event_id on the Pixel, or an event_name case mismatch (Purchase vs PURCHASE).

How do I fix it? Generate one event_id client-side, send it with both events, keep event_name identical, verify in Test Events.

Sources

Meta — About event deduplication (Business Help)
AGrowth — Event deduplication in Meta Ads
SignalBridge — Event deduplication
Analyzify — Event deduplication for Meta Conversions