Steps to fix duplicate empty vendors:

  1. Search for vendor name.
  2. Investigate what data is on each vendor (contacts, notes, items, brands, collections, NOT relationships nor access policies).
  3. If there are blank vendors, or vendors whose data is on other vendors of the same name (all contacts / notes appear on the others) these should be deleted.
  4. For vendors with less data rename the vendor and change the slug (click on edit vendor to edit the fields).  I've generally been using "(Use other)" as a name suffix, and "-dnu" as a slug suffix for vendors.