You might be able to use conditional formatting to at least highlight if the
number has already been used e.g.
select cell A2 and then choose
Format ->Conditional Formatting and select formula is and enter the
following formula
=COUNTIF(A:A,A2)>1
an format pattern as red
Copy this format down to the area you require , and any duplicates will be
highlighted as red.
Regards
John McC
Tipperary