Assuming that all numbers are in column A and begin at row 1 and go down to
row 1000, you could put this formula into a cell on row 1 and extend it down
to the bottom of the list and it will show a value when the number on that
row is in the list more than once:
=IF(COUNTIF(A$1:A$1000,A1)>1,COUNTIF(A$1:A$1000,A1),"")
Make sure you use the $ symbol in the places I did.