The Client Name field in Tabs3 is linked to the Contact file. When entering a new Client Name (instead of selecting an existing one), you are also creating a new Contact ID. However, Contact IDs must be unique, a lot like Client IDs. Therefore, if you enter the name of a contact that already exists, a new contact will not be created.
However, there are ways to add to the Contact ID in order to create a unique record. A few of the most common ones are as follows: