Why does Google Tag Manager say my GA4 configuration tag is firing twice?

Duplicate GA4 configuration tag fires happen when you have hardcoded global site tags (gtag.js) active in your theme files while simultaneously firing the exact same configuration tag via your GTM container.

“This is a classic case of a hardcoded gtag.js snippet still living in your theme while GTM fires the exact same tag on top of it. I’ll remove the duplicate and have it firing once within 24 hours for $99.”
— Jeff Bollettino, Managing Director, Ignite Conversions