That second, irresistible slice of chocolate cake is almost always too much. It tastes good, of course, but the pleasurable surprise has dissipated. A sated palate turns discerning — the frosting seems a little too sweet, the base too rich.

And yet somehow, there is an appetite for thirds.

The same can be said for the second season of “Downton Abbey,” which returns on Sunday to PBS with a flush of excitement and heightened expectations usually associated with “Mad Men” or even, back in the day, “Dallas.”

This elegant soap opera about masters and their servants in the twilight of the British Empire was a shameless throwback to “Upstairs Downstairs” and “The Forsyte Saga.” Season 2 is in many ways as captivating and addictive as the first, but this time around, the series comes off as a shameless throwback to itself.

The creator, Julian Fellowes, conceived “Downton Abbey” as a mini-series, but the viewer response was so enthusiastic that he and the producers decided to add more seasons. Accordingly, it’s a sequel that feels like a prolongation: plot twists are repeated, and the same devices are used in too many scenes. (Nosy servants overhear every private conversation, and nobody ever learns to close the door or talk outside.)