Duplicates can be caused by syncing both an API and a CSV at the same time for one exchange — like connecting an API for Binance and uploading your Binance CSVs as well. If this is the case, delete either the API or the CSV connection, so that you only have one data set in your account.
If you are adding new CSV records, be sure to only export the date range of your new data, or trip older data already in your TokenTax account from the CSV file using spreadsheet software like Excel.
If you have prior data uploaded as a CSV but want to add in new data with an API, be sure to set a start date on the API so that transactions only sync from after the final CSV transaction. That way, you will avoid API/CSV duplicates.
If TokenTax support has filled in prior years of your account history, be sure to set a start date on APIs and to cut off prior data from CSVs you upload so as to avoid overlap.