hello everyone

What I have is a sheet which is called "Credit Slip No" with all numbers from 1 to 1000 in Column A, now when they enter the Voucher number on any sheet (which may end up being lots of sheets) all the information pertaining to this Credit Slip No is then recorded in the sheet "Credit Slip No". Now my question is. Is it possable to prevent any person from entering the same Credit Slip No more then one time on any of the other sheets beside the sheet called "Credit Slip No". This Credit Slip No when entered on the other sheets is always entered into column A. So in actual fact the number is already in Sheet "Credit Slip No" in column A so now it can only be entered one more time on any of the other sheets. I hope I have not totally confused everyone with my explanation.

Paul Maynard
Moscow
Russia