There is a new error message that has been officially introduced for Google Merchant Center merchants. The new error is known as "Duplicate offers" and shows up when Google thinks "there are duplicate items in your data feed that should be removed."
Google's Ronald from the Merchant Center team said in a Google Merchant Center Help thread:
As some of you may have already noticed, we've introduced a new error message called "Duplicate offers" in the data feed details page. This error message suggests that there are duplicate items in your data feed that should be removed. Please know that the Google Merchant Center program policies do not allow posting of duplicate items.
Some have indeed already noticed. You can just Google duplicate offers and restrict it to Google matches and you will see tons of complaints.
I assume Google has let the policy slide, but according to the policy:
Duplicate offers within the same account, or between separate accounts that you control, are not allowed.
Forum discussion at Google Merchant Center Help.