I've got a sitemap that is generated daily with a lot of links to product pages. These products are imported daily from another data source. Because the update consists of throwing away all current product info and replacing it with the new imported info the last modified date always jumps one day. This is also used in the sitemap. Even for products that haven't changed. All product pages pretend to have been updated.
Will Google penalize the website for pretending the pages have changed from day to day while they haven't?
My solution would be to only change the entry only if the new imported product data differs from the previous data. I just want to make sure this is a useful upgrade to make, while I could also spend my time on other improvements.