I'm sure I should know a better way but right now I can't recall it. Soooo ..
One approach would be to create a Summary with your two columns, set the SSN column to NOT display repeated values and also sort everything by date descending.
That should give you a summary that only reports the first occurrence of each SSN and associated with that line should be the most recent date.
Export the summary (or cut and paste or whatever is appropriate) and create a new file. You can make this your preferred file type - fixed width text or csv/Excel or whatever suit your purpose.
You can now filter it, reading it with Monarch if it is a fixed width report or something that Monarch can read as a database, to exclude any rows that do not have an SSN. (Or, if it is a report read into Monarch, only select (by trapping) those rows that HAVE an SSN. You may be able to use your original model for that ... depends how you can create the trap.
Does that help at all for now?