Well it is not that Google doesn't care. If the Google feed expects certain field to hold certain value it will complain if it doesn't. However UPC and EAN in general represent unique article identifier. They are used in different parts of the World and sometimes interchangeable. I think. So Google asks you to have 2 out of 3 identifiers and whether you supply UPC or EAN code won't matter.
And yes this product identifying code must be unique. You cannot have 2+ products with same identifier.
- search for "gtin"