If you have numbers in cells A1:A100 and you want to find which are
repeated, in B1 enter
=COUNTIF($A$1:$A$100,A1)
and copy down
This will show the number of times each value appears in the list.
If you just wanted to highlight which rows had repeated numbers then
=IF(COUNTIF($A$1:$A$100,A1)>1,"Repeat","")
will show the word Repeat alongside any numbers occurring more than once
in the list.