Excel mark duplicates.

Simple and interesting:
- in one helper column concatenate the columns you are checking for duplicates.
- in another =COUNTIF([concatenated_colum_range];checked_concatenated_value)>1
It returns TRUE when there are more than 1.

Found here: http://www.ozgrid.com/forum/showthread.php?t=59302 and http://www.ozgrid.com/Excel/highlight-duplicates.htm and http://blog.contextures.com/archives/2013/04/11/highlight-duplicate-records-in-an-excel-list/