Assuming you have your numbers in Col A from row 1 to row 20 then
Enter this in B1 and copy till B20
=COUNTIF($A$1:$A$20,A1)

Duplicate numbers will have a count of 2 or more against them.

"Manni" wrote:

Hi,

I have a data I'm working with and I want to create a formula to tell me if
the number I am working on is a duplicate in that same data. Can someone
please help?

Thanks.