no repeats in a column

[repeat]

I have social security numbers in a column. I cannot have a repeat of any of
these numbers. Is there a way to set it up so it alerts me if I have two of
the same social security numbers in that column?

 

[N Harkawat]

select the column where you will enter the SSN
go to Data Validation and enter
=COUNTIF(A$1:A$1000,A2)<2
this will give an error message whenever a SSN is repeated.

 

[mzehr]

Hi
Try:

Use Data Validation--Settings--Allow: Custom--Formula:
=COUNTIF($C$1:$C1,C1)=1