PayPal error "Payment has already been made for this InvoiceID"

If using PayPal from multiple sources, it's possible for PayPal to incidentally get the same invoice number from multiple sources. When this occurs, customer's won't be able to check out on the site, and will be presented with the following error:

In those cases, you will just need to adjust the PayPal invoice number to allow for any extra orders and avoid the order from being rectified. This is done directly within your PayPal account.

Resolving the issue: 

• Log onto your PayPal account
• Navigate to Profile > Selling Tools
• Next to the section "Block Payments", click on the Update option
• Change the setting "Block accidental payments:" to be "No, allow multiple payments per invoice ID"

Save the changes to resolve the issue.