Do you watch different series in DVD's all at the same time?

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DO you finish a series in DVD then buy another series after finishing it or do you watch it all at the same time. I'm watching grey's anatomy this time. There are other available DVD's here in the house like Heroes, prisonbreak, greeks but I prefer watching grey's alone for the time being until i finish it then i will wtahv heroes and finish it again then the next. How about you? Do you prefer to watch 'em all at the same time even if you haven't finished a whole series yet?