I have been using the Append/Update Data method for a little while now and have not run into this issue yet. My excel spreadsheet has a unique numbers field that matches one of the fields I have set as unique in my hosted feature service. The spreadsheet also has 30 fields with the same field name. The spreadsheet does not contain every single field that is in the hosted feature service, but this has not mattered previously. I'm unsure of what is going wrong.
Here is the hosted feature service: ParcelLandGrade - Overview (arcgis.com)
Attached is the spreadsheet I am trying to use to update.
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