Thanks for stopping by the Community, I see what you're saying, I just tried to create a Vendor and a Client/Customer in our QuickBooks Online Test drive tool, and if they're the same person you get the following error. This name is being used by another vendor. Please use a different name. Here's how you can edit and change the display name:
Sign in to your QuickBooks Online company file.
Select Expenses, then Vendors.
Select the name of the duplicate vendor.
Select Edit (Note: Select Sales and then Customers if you are doing this to check for duplicates in your customer list).
Change the Display name as format to "last, first".