Custom Variable Template for Google Tag Manager
Generate unique event ids for deduplication or event tracing.
The template can be used to create a uique event id at every event-push to the dataLayer in Google Tag Manager. Use the variable for deduplication of FB hits from browser.
Add the variable as eventID to custom HTML code (https://developers.facebook.com/docs/marketing-api/conversions-api/deduplicate-pixel-and-server-events/) or "Event-Id" field in the FB tag, when using the facebookincubator tag template.
Then add an event parameter event_id to your corresponding GA4 tag that fires on the same event.
Note: If you are not able to fire both GA4 and FB Pixel on the same dataLayer event, you can still use this variable but you will have to provide a custom event counter instead of gtm.uniqueEventId from the dataLayer that changes with every push. For example this might occur when different trigger groups are used to combine events and vendor consent. In this case you can use either a custom event counter that is independent from the dataLayer or use constant values like a transaction id as a custom counter and multiple instances of this variable (one for every event type with a different custom counter or constant event name like "purchase").
This matches a workaround using a cached event id for every event type provided by a custom JS variable. There is a gist with a workaround to create such a cached event id for every type of event (and some fixes for other edge cases) at https://gist.github.com/mbaersch/5919bc1c795031183a525cb8c64a9352