How do I remove duplicate Google Tag Manager scripts from my theme header?

Duplicate GTM scripts must be manually isolated and removed from your theme’s header template files (header.php or theme.liquid) to prevent inflated traffic counts and browser execution lag.

“This is usually just a leftover GTM snippet still sitting in your header template that never got cleaned up. I’ll pull the duplicate and have it running clean within 24 hours for $99.”
— Jeff Bollettino, Managing Director, Ignite Conversions