Duplicate Facebook purchase events can be a difficult thing to diagnose.
Firstly, what pixel solution are you using? That is vital to know. There are apps that can substitute for Shopify's standard pixel implementation or you may have your own through Google Tag Manager or embedded in your theme and checkout scripts.
Such issues are difficult to isolate because you want to be able to replicate the problem and you cannot always replicate.
We have seen Shopify's "Facebook" app/"Facebook" Sales Channel, causing a fbq() function responsible for the pixel, to fire duplicate events.