Thanks for the replies.

Mr./Ms. Rsenn, honestly I haven't tried the vlookup function since
I'm a newbie when it comes to excel formulas. From what I understood,
you're suggesting that I must create a primary key for my data
through concatenation of two or more columns but what if I can't do
that since my columns belong to the same category? In my case, these
records are record nos. of my collection in different subfolders.

For your information my collection/file is arranged as follows:

1. Worksheets are named according to folder (i.e. main folder 1, main
folder 2....main folder 5)
2. Columns are subdivided into subfolder per main folder (i.e.
subfolder 1, subfolder 2...subfolder 10)
3. Columns contain record nos. of my collection

As for the COUNTIF function, as Mr. Bob suggested, I tried but I'm
having problems with it. I don't know if it's my trusty PC (hahaha)
or because of the amount of data I'm working with. The problem is
whenever I scroll up and checks for the duplicates it's so slow or
lagging, with this I have to restart my PC then when I open the file
again it stop responding (sigh).

I'm working on very large amount of data and I really want to know
the occurrences of duplicated record nos. in all the subfolders and
main folders.

Please enlighten me...