I think I'd do the copy|paste, but then use a helper column of cells that showed
me the duplicates:

Say column A was used to check the entries in column B.

=COUNTIF(B:B,B1)

Dragged down column A. Then filter on column A to show greater than 1.

And you may want to take a look at Chip Pearson's site.
He has a bunch of techniques for working with duplicates at:
http://www.cpearson.com/excel/duplicat.htm

BenBlair wrote:

We have a list of about 8600 different numbers which are our accounts. Each
is 16 digits long. We have recently had the problem that when we copy over
account lines, sometimes we create duplicate cells. We need to prevent this
duplication. Is there any way in Excel to prevent the addition of cells that
have the same content?

--

Dave Peterson