A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data
I suspect that you accidentally selected multiple sheets (by Ctrl-click on sheet tabs) and then enter data into one of them. All other sheets will then get the same entry in the same cell.
More about that in the link below.
https://support.microsoft.com/en-us/office/select-worksheets-096b40c9-0ee7-4980-bac6-cc92aec7b266