I don't really know this since I don't have monte vista, but is it working like riverview? I mean like you download it from the net and then install it?
If yes what you have to do is (in case your computer/laptop is win 7 operated) open :
Documents/EA/The sims 3/World Caches
then delete the extras and just leave one of the sims and objects package. If you just have one set of the sims and objects package that means you should have just one montevista world then.
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But in case you use Mac or other prototype, then I probably can't really help much.
I know this from my own experience. I did installed my riverview twice and result in double world.