Duplicate a layout that currently lists all of your samples, including the duplicates. On that copy of the layout, use the 'Add a Layout Part' tool to add a sub-summary part below the Body part. You will be asked to define what causes the sub-summary to 'break'. Choose the field that defines the duplicate - be that Rock Type, Mineral Name, or whatever it is that defines it as a duplicate. Put a copy of that field-that-defines-the-duplicate into the sub-summary part.
Return to Browse mode and sort all of your records by that field. You will see that all records are listed, including all the duplicates. The list is punctuated by breaks after each Mineral Name (if that is your definition of a 'duplicate'.)
Now go into layout mode. Delete all the fields from the Body part, and slide the body line up to meet the header (or the next layout section above it). In other words, delete the Body section completely.
Return to browse mode. You will see that you now have one line per mineral name (or whatever).
Thank you for the quick response and thanks so much for the information. It worked like a charm!
Another approach is to create a table of minerals linked to your table of samples. Such a table would have just one record for each mineral and thus also produces the results you want here. Both options are useful and are not mutually exclusive. Often, you need such a table to make it easier to work with your database and often the summary report Sorbsbuster has recommended is useful for displaying statistics about your samples--such as how many samples of each mineral is in your collection.