Find and mark duplicates

[maxtrixx]

If I have two columns with data, how do I compare these two columns to find
all duplicates and put an X on a third colum next to all duplicates? Thanks.

 

[N Harkawat]

=if(a1=b1,"X","")

 

[maxtrixx]

The problem is, I don't know which two cells to compare, I need to know if
any of the 800 items in the first colum exist in the 600 items of the second
column. It's not a row by row comparison. Thanks.

 

[David McRitchie]

hi ...maxtrixx...
You are correct that answer will not work, I suggest you
look at Chip Pearson's page on duplicates.
http://www.cpearson.com/excel/duplicat.htm#TaggingDuplicates

But here is the formula, which you can fill down with the fill handle.
C2: =IF(COUNTIF(B:B,A2),"x","") -- column A has a duplicate
D2: =IF(COUNTIF(A:A,B2),"x","") -- column B has a duplicate