You need to be careful here. In order for this to work seamlessly the client name in QF must match “to the letter” with the way the client is named in PayPal. Obviously this will often not be the case so there’s a high probability you’ll end up with duplicates.
If you import your own client list you’d need to cross-check it with those clients already there. You could run the import then order all clients alphabetically as a quick and easy way to isolate and remove duplicates.
After running the import you may also want to switch off the auto-tagging feature in PayPal and manually reconcile to your own customer records.