How are we supposed to drive on Brookline Blvd?

At first, I assumed, and most drivers seemed to agree that there was one driving lane and the right lane was for parkers to get in and out.  But, now people are regularly driving in the right lane behind the parked cars.  At the Cannon, if you want to make a right, you have to move over into the right lane early on or people will block you if you try to go from the "driving" lane to the left.  What is correct?  Is the left lane a single driving lane or should we drive in both lanes?