I've seen this once or twice here.  I had to use maintain database to delete the multiples.  I never did figure out what caused it.
 

   It was a while back, but here are the files:
 
Sales order header is BKARINV.
Sales order line items is BKARINVL.
Sales order notes is BKARDESC.
 
   There may be others, depending upon if the dupes were complete sales orders or just the header info with no line items.  (I think I had some like that).
    If they were added to the end of your last real sales order, you may be able to change your next Sales Order number in SD-M so that you don't have a gap.
   Just to be on the safe side,  I would back up the data before poking around in the database files, that way if something goes wrong, you can restore with no harm done.
 
David