Multiple markups on a single product page

If you are interested in a way to avoid the uncertainty (as Google will most likely never document what they’ll do in such a case, and it could change anytime, or depend on additional signals etc.):

Give both Product items the same URI as ID. This conveys to consumers that both of the Product items are about the same product, not different products. Here is an example with all three syntaxes (JSON-LD, Microdata, RDFa).

(Of course, the ideal solution would be to have only one Product item in the first place, but this is often impossible or hard to implement.)