Custom metric - Started Checkout in GTM 

A plugin was used for the integration between Shopware & Klaviyo. The checkout process is not recognised by Klaviyo. Therefore, I implemented a custom html script via Google Tag Manager to trigger the Started Checkout metric in Klaviyo for the cart abandonment flow.  I used the following script using this article:

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<script
type="text/javascript"
src="//static.klaviyo.com/onsite/js/klaviyo.js?company_id=xxxxx"
></script>

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This script works, but in the Started Checkout metric in Klaviyo, it fires 5x. How do I make sure it only fires once?