It look like you are trying to put <PAGE> entity in your excel spreadsheet which obviously has to have unique values. Do you have one Sheet in your Workbook or multiple. If multiple - then maybe one of existing sheets has the same name as the value under the field you are using for <Page> entity?
putting bigger snapshot would be more helpful with:
- sheet names visible
- entities visible
- field values from field used in PAGE entity (if you are using it)
we will then take it from there.