After installing JSON-LD for SEO and checking your product pages in Google’s Structured Data Testing tool, you may see results returned for multiple products, like this:
The most likely cause of this is that your theme’s page markup contains existing structured data markup in the form of Microdata.
Don’t worry if you see this.
In almost all cases, Google is able to recognize that the two sets of structured data relate to the same product. They will simply use the most correct and complete version of the product in their data analysis and ignore the others. If there are inconsistencies in the product data or something else is wrong, you’ll receive a warning about it in Google Webmaster Tools.