When buying a duplex if you would like to occupy both halves, you will want to look for a vacant duplex. This will really be the easiest avenue for you. When a property is tenant occupied, you will need to make sure that you follow the guidelines of Berkeley's Rent Control. I would really suggest you check out Berkeley's rent control information so you have a better understand of tenants rights when a property is being sold.
A buyer does have the right to evict tenants for owner occupation, but in the case of a duplex it can be more complicated becuase there are two units.
Here is a link to the City of Berkeley's web site with the information regarding rent control: http://www.ci.berkeley.ca.us/ContentDisplay.aspx?id=5668
Often times in the instance when the buyer is looking to evict a tenant for owner occupation, hiring a lawyer who specializes in this process is a good idea to hire just to ensure that the process is completed by the letter of the law and to ensure that the tenant rights are not violated.
Alain Pinel Realtors