Thanks everyone! We really didn't want to set a unique field of any sort because of the ability to get around that, and we really don't want a hard stop for any reason. So, instead of a pop up message, we created a table that checks for duplicates and the records are populated from a triggered action on our calendar table using an inner join on the two tables. When that condition exists, a record is added to the dupes table and our managers can run a report to see if there are any potential duplicates. Then, they check off that they've verified the record and that record is deleted. Not perfect, but definitely does the trick.