Try this:

Entered as an array using the key combination of CTRL,SHIFT,ENTER (not just
ENTER):

=INDEX(A$1:A$4,SMALL(IF(ISNUMBER(MATCH(A$1:A$4,B$1 :B$4,0)),ROW(A$1:A$4)-ROW(A$1)+1),ROW(A1)))

Copy down until you get #NUM! errors meaning all matches have been returned.

Biff

"HERNAN" wrote in message
...
I have 2 columns with numbers Column A and B.
I need to COPY on column "C" the ones that are duplicates.
That's it.
(I don't need to find them using conditional formating, I need to copy
them)
Please Im gettin crazy with this. THANK YOU IN ADVANCE!!!!

A B
10 30
15 50
20 15