this is easy
let's say column A is your column with all the numbers in it..
in column B, put a 1 next to each and every entry in column A
make a pivot table of the data in columns A and B, and for the row header choose the column A number, and for the data section choose Sum of B, and sort the whole thing in descending order...any dup's will have a total of more than 1 and be at the top of your list...
you could also do an advanced filter on the list, and select unique records only, and put that outcome in another column....then see if the lengths are different - but that would only tell you if any dup's exist, it wouldn't identify them for you