Somehow we have two POs going to different vendors that have the same PO number.  Can anyone tell me how this might have happened and what I can do to make sure it doesn't happen again?

Yes, we have seen this twice. In both cases it was when two people were adding POs at the same time. I think the incrementing sequence has a small window of opportunity for error. Perhaps it grabs the number during PO entry, but does not increment until the PO is saved?  
 
In both cases we have left it and just processed the receipts and invoices normally and everything has worked out, with the exception that the vendor associated with the PO winds up being the last one for whom processing was completed  

Is this with  T7POA, T6POA or BKPOA?

T6