validating format

[oldLearner57]

hi community

can anybody assist me to perform this task...

A B C
1 1012345
2 1012346
3 1012347
4 1012348
5
6

base from the above simple scenario, is there a way to validate the column A
that if I enter the number say, 1012345, there is some kind of indicator,
maybe it turn to a color (like red color 1012345) and this will assist me to
know that it is a repeated number?

I already had 1500 rows that already have number entered.

thanks community for the kind assistance

 

[David Biddulph]

Conditional Formatting/ Formula Is/ =COUNTIF(A$1:A$1000,A1)>1

 

[Rick Rothstein \(MVP - VB\)]

If repeats are allowed and you just want to know about them, then David's
solution is what you want. However, if you want to be alerted to, and
prevented from entering, duplicate values in Column A, the select the entire
column, click Data/Validation on Excel's menu bar, choose Custom in the
Allow box and uses this custom Formula...

=COUNTIF(A$1:A1,A1)<2

Rick