Mark all duplicates in excel 2007, libre office, and wps spreadsheet.

in 2007 excel to find duplicates:
-select column with duplicates
-Home > Conditional Formatting > Highlight Cell Rules > Duplicate Values

libre office calc
-select column with duplicates
-Format /Conditional Formatting/cells
-select "Cell Value is" in first combo box
-in the second combo box, select "duplicate"
-select some style, or click on "new style", and change cell background setting (in new window.)
(-you can change range at the bottom of the window, if you wish)

wps spreadsheet/
-open new empty excel document
-copy column with duplicates to new new excel document, in column "A"

-select column with duplicates
-Home / Format /Conditional Formatting
-select "Formula is" in first combo box
-in the next text box enter "=COUNTIF($A$1:$A$100,A1)>1" without quotes
-replace 100 with the number of rows in your data.
-click on format to change the way duplicates are marked.
-you can chage font color to red, or change cell background to some color to see the difference.