If you have a relationship between the tables:
in Person (table)
Count ( Jobs::jobID )
this is a vague answer, I know, without more information (screen shot of your Relationship Graph perhaps showing the link between the two tables?)
Counting duplicates is super simple whether or not you have a relationship to another table.
Just create a calculated field and use the SQL below or something that matches your structure.
(** Click on the image below to see all of it **)
If you really only want a count of the duplicates, then subtract 1 from this total count in your calculated field.
HOPE THIS HELPS.