I have to do somthing similar, what I have done is
Create a report with just the one field in it
Group by the field so that all records of the same value are together
Add a count of records per group in the group header
Use a group selection to show groups where the count is greater than 1
This should give a list of duplicates, if you want you can supress your detail and group footer, this will make the report look neater and will only show the value once.