In order to avoid entering the same person twice, you are required to search for somebody before you can add them to the system.  To do so, you type at least 2 letters into the search box shown below.  This searches on the Last Name. Doing so will show any matches and, if the one you are looking for does not exist, you can then add the new person immediately.

When you press the Assign to current button, the program will ask you to confirm only if that person is already associated with another law firm.

Make changes to key information if necessary by clicking into those fields.