Re: Duplicate item error
Duplicate Item error means that it looks like some issue with the variant level data which you have added in your feed. If you have added several variants of the same product as separate listings, this would be considered as duplicate and you will get errors in your feed. So you need to fix it before uploading the feed again.
More feed related specifications are mentioned here: https://support.google.com/merchants/answer/188494?hl=en