Last night I was playing a saved game, with only one actively played family. As is my habit, I had my Bridget read the paper and it said that a new family had moved into the neighborhood... The Mitchells (a family I had created and added just to have them).

Nothing odd about that except that Bridget was living next door to them. I went to Edit Town and went searching and found them surely enough on the far side of town.  They were exact clones of my Mitchells next door, down to traits LT Wishes and all.

So I am thinking one-time glitch. Not so. Later when playing a Multi-Family neighborhood where I was rotating between 3 families every 4 Sim Days or so, I read in the paper that the Petersens have moved into the neighborhood. Now the Petersens are one of the three families I am rotating.

Not that I mind new families moving in and keeping things lively... I do NOT want clones. I do not want to waste my time at the park building relationships etc only to find out that they are the CLONE and not the original. Is there anyway to prevent this moving in of existing families or Sims. And, what is the best way to rid myself of the ones that have already spawned in my 2 neighborhoods?

Ya, that is how I handled them.  I was just really really hoping that there was a way to prevent it in the future.  It is really disheartening to work on a relationship only to find out that you were chatting up a clone.

--crosses fingers for an Awesome Solution in the future--

You can delete families from your Library (neighborhood bin), otherwise the game considers them as an option to move in as a "new" family.
If for some reason, you want to keep a copy, backup your Documents\Electronic Arts\Sims3 \Library folder and then delete the contents.