Excel 2007 : unwanted copying of text from a cell to the corrosponding cell on all sheets

I have a smiple spreadsheet, with no formulars. We run courses and each sheet within this excel file represents a different course (12 sheets total) the information stored is of registered participants eg. A1=name B1= School C1= phone number etc. and the information for each participant is recorded beneath these headings.
After sometime working with this spread sheet I have opened it to find that cell A15 and A18 from the 8th sheet have copied themselves to A15 and A18 of every sheet! if there was text in this cell it replaces it. this is quite a bother and happed previously with this spread sheet though it was whole rows that were being copied accros the file.
I did not create the original spread sheet so, for fear there may be forumlars I could not see, I printed and retyped the whole thing, no copy and paste so that it would be nothing more than text. but it is happening again.
Does excel go funny if you have too many sheets within one file? could having dates and phone numbers recurring in the sheets be confusing excel? I have tried to get it to show me the formular that may be causing this problem but can see none. the IT department at my work is stumped. I am confused. any help or advice would be most appreciated.

Hi sarah.perkins1

It is due to the grouping of the worksheets in the file. i guess you mistakenly group the sheets in your file.

Kindly read "Select one or multiple worksheets" from excel help.

Kindly note that if the sheets are grouped, Data that you enter or edit in the active sheet (active sheet: The sheet that you're working on in a workbook. The name on the tab of the active sheet is bold.) is reflected in all selected sheets. These changes may replace data on the active sheet and, perhaps inadvertently, on other selected sheets.