I would do this in Access (after importing the spreadsheet).
Make a group-by query (grouped on all fields except salesperson). Add a count field. Select if count > 1. Voila.
The problem, in practice, will be spelling variations and typo's (like McDonalds and MacDonalds and a lot of other ones); those aren't counted as duplicates. So some (I mean: a lot) of additional work lies ahead: sort on state, postal code, company name, person name and do a manual compare in printed reports. You'll never catch everything, but you could get a rather clean database given careful work.
In fact, I would never use a spreadsheet as a database used by different people to maintain their contacts. Consider to switch a simple multi-user CRM-system, so yoy can have shared contacts, shared notes, shared orders and shared everything between the salespersons.
But that's your choice, of course.
Here is the problem: I have an excel database of 10,000 leads input by various salespeople--name, address, title, phone are the column headings, along with a column heading/field for the responsible salesperson.
It just so happens that many of our salespeople have the same contacts input into the database. The only difference between the records is the responsible salesperson. We estimate that approximately 5% of this database are dupes.
How do I use Excel or Access create a report or isolate just the duplicate records so our sales team can figure out how to clean up the database?