Please help! Conditional formatting & filtering

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I'm a total novice when it comes to excel. I have 3 columns of data. In column A & C, I have lists of account #s. I have an ID # listed in column B. I applied conditional formatting in columns A & C to identify duplicates. What I need to find now is the value in column B, whenever the value in column C is identified as a duplicate. Problem is, I have over 120k rows of data so I can't filter! Hopefully this makes sense, would really appreciate any help!!
1 Reply

@Kfelux If you have dynamic arrays then I think this will work:

=FILTER(B:B,ISERROR(MATCH(C:C,UNIQUE(C:C,FALSE,TRUE),0)))

or without this should work if you paste and copy down 

=INDEX(B:B,AGGREGATE(15,6,ROW(C:C)/(COUNTIF(C:C,"="&C:C)>1),ROW(A1)))

 NOTE in both cases performance can be improved if you use actual ranges instead of the entire column.