we have had (in a couple of cases already) a problem when submitting forms from ODK collect 1.30.1 to ODK Central 1.1.2.
The forms should be finalized (and submitted) automatically, but sometimes this does not happen immediately (due to connection), and when the enumerator tries to force the submission, he/she got the error message:
Generic Exception: Error: A submission already exists with this ID, but with different XML. Resubmissions to attach additional multimedia must resubmit an identical xml_submission_file.
It seems to be related to this this post but it has a slightly different error message.
One possible explanation (that I can think of) is that the submission process was somehow interrupted during the upload, so that ODK Central received some metadata, but not the complete form. Is it possible?