Identifying Duplicates in 2 columns

[Excel 2007 Newbie 2]

I'd like to identify duplicates in columns A and C with "xxxx" in column B.How can I do this?
I would then know how to sort Z to A and remove duplicates.
A B C
1 123 Main xxxxx 123 Main
2 135 Elm 145 Oak
3 178 Bell xxxxx 178 Bell
4 194 Maple xxxxx 194 Maple
5 256 Cedar 275 Cedar

[6StringJazzer]

Do you mean put xxxxx in column B in any row where column A matches column C? If so, put this in B1 and copy down:

=IF(A1=C1,"xxxxx","")

What do you want to sort on, and you want to remove duplicates of what? Do you mean sort by column A and then remove any lines that have xxxxx?