Vendor bill import not allowing duplicate doc numbers any more

Prior to the new release it was possible to import vendor bills with the same reference number, as long as the external ID was different.  We are a distribution company and often name AP bills something generic like "August 2014 sales" so that when we generate searches we can find all of these documents simply to get a net sales total for the month.  And more simply, it is possible to receive bills from different vendors that have the same number.

Now, with the new release, when we import bills of this type, I get the following error message in the CSV response file: "A record with this ID already exists. You must enter a unique ID in order to create or update this record."  HOWEVER, if I manually enter two different bills for two different vendors using the same reference number, I get no such error.  This problem did not occur before the newest release...any ideas on how to fix this would be welcome.  Thanks!