I'd assume you have the code in place twice but without being able to see the code that's a pretty bold assumption.
I had the same problem -- we had our pixel inadverantly placed twice. Once in the new Facebook Pixel Code Implementation field (the HTML-less feature where you just add your Facebook Pixel ID) and once in the additional scripts field. We removed from additional scripts and the double firing stopped.
Our issue is that now that we're using the Facebook Pixel Implementation option, there's no way for us to add first_time_accessed tags, so the conversion pixel fires when someone purchases and every time that they check their order status.
Does anyone know how to use the new Facebook Pixel Implementation field AND add a first_time_accessed tag, so we can use the automated integration without having the conversion pixel fire on the check order status page too?
Many people have solved it. You will have two instances of the pixel firing. Most people use Shopify's native Facebook pixel integration. Which is what most stores need. You can copy the pixel ID then search it in your checkout scripts. Another common place it can fire again is in Google Tag Manager.