Why am I seeing a 403 Forbidden - DUPLICATE_SUBMISSION error?

The reason you are seeing a 403 Forbidden - DUPLICATE_SUBMISSION error is because you tried to submit a VAT return that has already been submitted to HMRC. The likely reason for this (although not the confirmed reason) is as follows:

Your initial attempt to submit the VAT return was successful but due to an unknown and hidden error, Easy MTD VAT was not notified of this and you were returned to the Submit VAT Return window without any indication of a successful submission. After clicking the Submit button again, the DUPLICATE_SUBMISSION error occurred because HMRC received the VAT return following the initial/previous click of the Submit button. You can check this by looking for the associated Fulfilled Obligation in the Easy MTD VAT interface. If it is present then you can be assured HMRC successfully received the VAT return. Due to the error, submission receipt data will not have been recorded.

If what described above is not what occurred, and you continue to see the DUPLICATE_SUBMISSION error, you will need to contact HMRC (see link below) so they can look into the matter.

https://www.gov.uk/government/organisations/hm-revenue-customs/contact/vat-enquiries