Hi Tao,
I've found that a short AppleScript provides a convenient way to identify duplicates (repeating values) in a range.
Just select the range of cells you want to check for duplicates and click. It will turn the duplicates red.
--select range, run, turns duplicates red
tell application "Numbers" to tell front document to tell active sheet
set selected_table to first table whose class of selection range is range
tell selected_table
set selected_range to the selection range
tell selected_range
set values_list to {}
repeat with i from 1 to count cells
set this_value to value of cell i as text
if values_list contains this_value then set text color of cell i to "red"
set end of values_list to this_value
end repeat
end tell
end tell
end tell
--end of script
Just copy and paste the above script into AppleScript Editor, and run it from there. Or place it in your scripts menu. Or download this Automator Service (Dropbox download) and double-click it to install it in your Services menu.
Similarly, if you want to get a list of just the distinct values in a range (i.e. with duplicates removed) this Copy Distinct Automator Service (Dropbox download) is very convenient.
Select the cells, make the menu pick, then paste the distinct values wherever you want in a table by single-clicking a cell and typing command-v to paste.
These functions become, in essence, a customized menu. That way you don't have to set up formulas to do this each time you work with a new Numbers document.

SG