If you happen to have several entries of the same customer/supplier in your list, you can merge entries together so you only have one entry per customer/supplier.
To do so:
Go to the Customers & Suppliers menu
Select 2 or more customers/suppliers then click on Merge customers (or Merge suppliers)
Pick the one you'd like to keep as the main entry
When done, all documents previously linked to the various entries will be linked to the main customer/supplier. Please note that merging customers/suppliers can't be undone.