Paypal: Duplicate Transactions

You must manually turn off the setting to block duplicate transactions.  

Steps to turn off the setting:

  1. Log into your PayPal account. 
  2. Go to Profile Subtab
  3. Go to My Selling Tools
  4. Go to Block payments or Payment Receiving Preferences
  5. Turn Off "Block accidental payments" => Prevent accidental payments by blocking duplicate invoice IDs