Why am I getting a large number of duplicates?

Only one possibility. You have another integration running next to PieSync for the accounts you have connected in PieSync. Other integration(s) will also have access to your data and the ability to manipulate. That's why we ask you, ever so kindly, use PieSync and nothing else to keep your data in sync.

Other integrations running in parallel with PieSync is a no-go!