Is it OK to use an oversized swamp/evaporative cooler?
I personally think that it's best to buy a cooler that's a little above the rated CFM you actually need -- this will help ensure it actually cools your house to a comfortable level.

Where I live in Albuquerque, NM, I often find myself in people's houses where the performance of the evaporative cooler is quite inadequate.  It's better than nothing, but not enough airflow to really be comfortable.  Having a cooler that's a bit over spec will help to prevent this problem, especially as the system gets older and performance may deteriorate.

This often causes people to want to switch to refrigerated air, which I think is sad, because it uses SO much more energy, and an adequately-sized and well-performing evaporative cooler can do the job well for a fraction of the energy use.

That said, having one twice spec is a lot.  I think you would be alright, though, as long as you keep in mind that you may have to increase the amount of window (or other) opening to accommodate the higher flow (and your ductwork is up to it).

From http://energy.gov/energysaver/evaporative-coolers: 


  Open the windows or vents on the leeward side of the house to provide 1 to 2 square feet of opening for each 1,000 cfm of cooling capacity. Experiment to find the right windows to open and the correct amount to open them. If the windows are open too far, hot air will enter. If the windows are not open far enough, humidity will build up in the home.


Also keep in mind you could always put in a smaller (lower horsepower-rated) motor in order to lower the effective CFM.