Duplicate contacts may be rare, since one may locate and attach a previously used contact whenever adding contacts to new jobs, for instance an agent whom you've worked with before. Also one might deliberately need to create a separate contact for someone who shares the same name, but is a different person or contact.
However if contacts were imported from another source, came from a previously converted database, or somehow were added again accidentally, you may manage your contacts to combine any duplicates you notice of the same individual. The combined contact will remain attached to the prior jobs.