How do I compare cells in a column

[Jim K]

I am inputing a large column of names and numbers and would like to insure I
don't repeat any. I'm not sure how the Vlookup works I continually receive
errors and the If command only works with 2 cells. Thank you in advance for
any help!!

 

[Max]

One way ..

Suppose the list to be entered is in col F

Select col F

Click Data > Validation

Make the settings:
Choose under "Allow:" : Custom
Source: =COUNTIF($F$1:F1,F1)<=1

Click OK

 

[Max]

Source: =COUNTIF($F$1:F1,F1)<=1

Minor typo: "Source: " should read: "Formula:"