Provisional applications in the US only last one year.  If no utility or international application is filed during that one-year window, a provisional goes abandoned, and cannot be used to claim priority.  Sometimes the applicant is not ready to file a utility or international application during that window, and instead re-files the provisional. 

The problem is that if that re-filed provisional is filed while the earlier provisional is still pending, there is a chance that a priority claim to the re-filed provisional will fail in a subsequently national or regional phase application.  Article 4(c)(4) of the Paris Convention provides that any subsequently-filed priority application can only serve as the basis for priority over an earlier-filed priority application if several conditions are met, including that there are no remaining rights from that earlier-filed application when the subsequently-filed application is filed.  In the European Patent Office (EPO), for example, a request for re-establishment could be filed up to 2 months after the one-year window has passed for the earlier-filed provisional.  If such a request for re-establishment is granted, there would still be outstanding rights from that earlier-filed provisional, and priority to the subsequently-filed application would fail. 

Best practice then, is that if a re-filed provisional is used as priority for a PCT application, any earlier-filed but still pending provisional should be expressly abandoned before refiling the provisional.  Kudos to Terri Shieh-Newton, PhD of Mintz-Levin for pointing out this problem.