Duplicates

Normally you can use something like

=COUNTIF($A$1:$A$10,A1)

which will give you the number of times something is in the list, or

if you want to list how many time the items is found

=COUNTIF($A$1:A1,A1)

this would tell you the first time an item is found and then number up from 1 if found more often.

Maybe some more information about what your data looks like or even attach a small sample workbook

both of these formulas are copied down your list starting in B1