Indicate First Duplicate Value in a Column when a condition exists in another column

[lager1001]

I need to indicate in the rows of column C where the the first duplicate value of Column A is when Column B is not blank. Any ideas?

See below example

[TABLE="width: 192"]
<colgroup><col width="64" span="3" style="width:48pt"> </colgroup><tbody>[TR]
[TD="class: xl64, width: 64"]Column A[/TD]
[TD="class: xl64, width: 64"]Column B[/TD]
[TD="class: xl64, width: 64"]Column C[/TD]
[/TR]
[TR]
[TD="class: xl63"]12345[/TD]
[TD="class: xl63"]abc[/TD]
[TD="class: xl63"]X[/TD]
[/TR]
[TR]
[TD="class: xl63"]12345[/TD]
[TD="class: xl63"]abc[/TD]
[TD="class: xl63"] [/TD]
[/TR]
[TR]
[TD="class: xl63"]12345[/TD]
[TD="class: xl63"]abc[/TD]
[TD="class: xl63"] [/TD]
[/TR]
[TR]
[TD="class: xl63"]12345[/TD]
[TD="class: xl63"]abc[/TD]
[TD="class: xl63"] [/TD]
[/TR]
[TR]
[TD="class: xl63"]6789[/TD]
[TD="class: xl63"] [/TD]
[TD="class: xl63"] [/TD]
[/TR]
[TR]
[TD="class: xl63"]6789[/TD]
[TD="class: xl63"] [/TD]
[TD="class: xl63"] [/TD]
[/TR]
[TR]
[TD="class: xl63"]6789[/TD]
[TD="class: xl63"]abc[/TD]
[TD="class: xl63"]x[/TD]
[/TR]
[TR]
[TD="class: xl63"]12347[/TD]
[TD="class: xl63"]abc[/TD]
[TD="class: xl63"]x[/TD]
[/TR]
[TR]
[TD="class: xl63"]88888[/TD]
[TD="class: xl63"] [/TD]
[TD="class: xl63"] [/TD]
[/TR]
[TR]
[TD="class: xl63"]88888[/TD]
[TD="class: xl63"] [/TD]
[TD="class: xl63"] [/TD]
[/TR]
[TR]
[TD="class: xl63"]88888[/TD]
[TD="class: xl63"]abc[/TD]
[TD="class: xl63"]x[/TD]
[/TR]
[TR]
[TD="class: xl63"]88888[/TD]
[TD="class: xl63"]abc[/TD]
[TD="class: xl63"] [/TD]
[/TR]
[TR]
[TD="class: xl63"]88888[/TD]
[TD="class: xl63"]abc[/TD]
[TD="class: xl63"] [/TD]
[/TR]
[TR]
[TD="class: xl63"]88888[/TD]
[TD="class: xl63"]abc[/TD]
[TD="class: xl63"] [/TD]
[/TR]
</tbody>[/TABLE]

 

[Fluff]

Assuming your data starts in A2, how about
=IF(COUNTIFS($A$2:$A2,A2,$B$2:$B2,"<>")=1,"x","")