You may easily find it out if you
- duplicate a submitted case inside the server account and wait a moment before saving/submitting
- download the Excel data for this new case and the original case
- look at and compare the metadata.
You may clone your project to focuse on these cases then.
Some hints can be found here https://community.kobotoolbox.org/t/multiple-submissions-randomly-appeared-all-at-once-on-server-days-to-weeks-after-submitting/31465/11
