Hi @dbareket,
According to PayPal, this error occurs when a PayPal InvoiceID value is already associated with another transaction in the PayPal system:
PayPal checks that InvoiceID values are unique for any particular merchant. If you send an InvoiceID value that is already associated with another transaction in the PayPal system, PayPal returns this error code. Inform the customer and redirect him or her to restart the Express Checkout flow. However, you might not be able to correct this error during an actual checkout. In this case, modify your implementation of Express Checkout to ensure that you generate unique invoice ID numbers.
https://developer.paypal.com/docs/classic/api/errors/#10412
Please check if you are using any 3rd party plugin that would interfere with the order number or order ID in your site.
