preventing duplicate enrties in a column

[Mark]

How do I make a rule that an identical number cannot be entered twice in an
Excell column?

 

[Bernie Deitrick]

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