@Neha.khanna has reported something very similar at 409 error while submitting a response. Could it be the same report by any chance?
https://github.com/enketo/enketo-express/issues/416 is the related issue that I had in mind. However, this looks different because it's the server reporting a duplicate. Did anything unusual happen when attempting to send? Have you seen this with multiple submission attempts or just one?
One thing you could try is to click the export button and find out the instanceID of the submission. Then you can look that up in the submission table on Central (using your browser's search, e.g. CTRL+F). You can edit the record from Central to compare the data there and in the exported record. Are they the same or different? Were they from the same person?