Mark,

Say in a very deep voice:
"Thou shalt not enter the same number twice in one column."
;-)

Actually, use Data / Validation, with a custom formula of

=COUNTIF(A:A,A2)=1

For column A, with A2 the currently active cell.

HTH,
Bernie
MS Excel MVP


"Mark" <Mark @discussions.microsoft.com wrote in message
...
How do I make a rule that an identical number cannot be entered twice in an
Excell column?