Yes you can. It is legitimate scenario for two agents to be working on the same contact. As long as you know at least the mobile number or email/home phone for that contact, you can search for the record using the search box and then add a new lead for that contact.
Agents cannot see each other’s leads. If you search for a contact that belongs to another agent, you will only see the contact information for that person, and not the previous lead entries for that contact.