Cassie from Google posted in the Google AdWords Help forums that those that use Shopify for their e-commerce sites and AdWords conversion tracking need to consider updating their conversion count to avoid unexpected duplicate conversion metrics.
Shopify made a change where users can now easily check their past orders by viewing their Shopify confirmation page. The Shopify confirmation page has the conversion pixels, which can lead to the pixel firing off and "double counting of your conversion within AdWords," Google added.
So these Shopify users should make an update. They need to update your conversion tags to include the Order ID. Please refer your web developer to these instructions for the proper setup of Order IDs.
Forum discussion at Google AdWords Help.