checking a sheet for identical items
 

Is there an easy and quick way to check a worksheet for identical items, like
customer numbers? My macro builds subtotals by customers. However, a user
may use 2 different sources to build these subtotals. If they use 2
different sources, it is possible to duplicate the subtotal in different
parts of the spreadsheet.

I would like to create some edit / process that would identify the
duplicates. I'm not sure if I should build an array of subtotals and then
check it against the other items in the array (not sure if this is even
possible) or if there is a better way.

Any help or suggestions would be greatly appreciated. Thanks.........
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JT