Duplicate items - formula to identify duplicates

[ExcelNovice]

I'm working with a fairly large spreadsheet.
Col O has data from cell O23 to O2500, most of which are duplicates.
I need a way to identify the first duplicate items in Col O, and I need the identifier for the duplicate to be in Col AG.
Example:
Col O23 CD250
Col O24 NT150
Col O25 CD250
Col O26 TG200
Col O27 DW300
Col O28 NT150

I need cell AG23 to indicate that Col O23 is the first duplicate for CD250. Ideally, it would be great if someone could share a formula that when entered in AG23, its returns CD250, indicating that it's the first duplicate of CD250 on the list. The formula result should then show NT150 in AG24, it would leave AG25 blank because it's not a duplicate, shows TG200 for cell AG26, blank for AG27 and blank for AG28 because it's not the first occurrence of the duplicate entry.

Hope above is not totally confusing; any help will be appreciated.

Thanks

 

[Aladin Akyurek]

In AG23 enter and copy down:

=IF(COUNTIFS($O$23:O23,O23)=1,INDEX($O$23:$O$28,ROWS($AG$23:AG23)),"")

 

[ExcelNovice]

Thank you thank you!
Works perfectly!

 

[Aladin Akyurek]

Thank you thank you!
Works perfectly!

You are welcome.