Regarding your concern that event is being triggered twice on your page
Reason: This is mostly because you manually added an event for your page, while your Theme automatically fired event before.
If you get the Theme from Themes Marketplace (themes.shopify.com), the reason may be that Facebook Pixel is already fired with “Add to Cart”. It leads to double-tracking when you manually fired Facebook Pixel.
If you feel my answer is helpful, like it or mark it as a solution. Let me know if you have any questions.
Richard | PageFly
PageFly- Advanced Shopify Page Builder - Empowering 100.000+ active merchants.