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Item has already been added. Key in dictionary: 'id' Key being added: 'id'

New Community Member

Item has already been added. Key in dictionary: 'id' Key being added: 'id'

We're using NVP/SOAP API integration on our platform, and have a number of different ecommerce sites using this method.  However, with one particular customer's API details in place, when we return to the merchant's site after completing the PayPal details, we are seeing the following returned - as far as we can tell - from PayPal:

Item has already been added. Key in dictionary: 'id' Key being added: 'id'

This drops the contents of the basket and we cannot proceed with the checkout process.  The API details seem to work correctly, in that they link us to the merchant's PayPal account.  We have generated new API details and get the same error, but if we use API details linked to a different merchant's account, we don't get the error.  When the error does occur, we get no trace of any errors coming from the ecommerce platform itself.

This points to a problem with that particular merchant's PayPal account, but I've compared their account settings to another account which we know works correctly in the checkout, and I cannot see any obvious differences.

It's not an error we've ever encountered before and there doesn't seem to be anything related online, so we're pretty stuck.  Any help would be much appreciated.