Bluebell, I've had this same problem. It was quite a while ago. Almost every time one of my Sims travelled, they would come home to find a clone in residence. I never did find a solution to the problem, except to save before travelling and, if I did save during the trip, I used "save as" to make a separate game file. Of course, you can just delete the clones but sometimes the Sim wound up with missing skill points and damaged relationships anyway. I don't know if this problem still exists in my game, as I basically stopped taking my Sims travelling because of the problems.