You are totally correct, trying to enter them again would create doubles. Follow these steps to correct the problem:
1. Go to Accounting-Names and Addresses-Add/Edit Address List 2. Show to box at bottom to Show Retired. 3. The missing Landlord names will display with asterisks. 4. Highlight and click the Un-Retire button. 5. Repeat for each missing landlord.
I do not have an answer as to what caused this, but this will correct the problem for you.