For ArcGIS, you will need to use 'summary statistics'.
That will give you a list of all the uniques, and a count of how many times each occurrs.
If you want to select for all the records which occurr only once:
Join the output table from the summary statistics table to the file, using the field you summarized them on as the join.
Add a definition query that says "Count=1".
(If you are working with geodatabases, instead put "Count=1 & IS NOT NULL".)
Variables that are unique will have a count of 1. Variables that show up more than once will have a count greater than 1.