You could use Data | Filter | Advanced Filter to obtain unique values
from your list (copied to B1 downwards, say). Then you could compare
the length of your first list with the length of your second list - if
the column B list is shorter then you have duplicates. To find out
which are duplicated, enter this formula in C1 and copy down by
double-clicking the fill handle on C1:

=COUNTIF(A$1:A$100,B1)

Hope this helps.

Pete