R
ritinha
Thanks Frank ... but 
I´ve tried the following and does not work ...
On "Data Validation" I´ve set the following validation criteria:
Allow: Custom
Formula:=COUNTIF($A$25:$A$25,A1)=1
Then, I´ve selected the "error alert" & introduced the warnin
message.
Then, I´ve pressed OK.
Then,
what happens ... any value is allowed ... when trying t
introduce any value in the column the error alert appears!
What should I do?
I would appreciate if someone could help me.
Thanks,
Ritinh

I´ve tried the following and does not work ...
On "Data Validation" I´ve set the following validation criteria:
Allow: Custom
Formula:=COUNTIF($A$25:$A$25,A1)=1
Then, I´ve selected the "error alert" & introduced the warnin
message.
Then, I´ve pressed OK.
Then,

introduce any value in the column the error alert appears!
What should I do?
I would appreciate if someone could help me.
Thanks,
Ritinh