This happens in my game a lot.  Not with all Sims, but most.  I always delete the second one, because it makes them look as if they had a lesser degree than they actually had.  I have no idea what causes it, but it's a pain, because the memory has to be deleted from all family members and anyone around at the time as well, otherwise it'll get passed on to all and sundry.

I currently have around 450 in Pleasantview and this is only with 2 expansions and also with not playing it for a while (because it's so full).  Bearing in mind all the extra characters each expansion is going to create, surely the game will allow for that?  If not, Maxis can't have intended anyone to use the same neighbourhood for the entire duration of the expansions because it sounds as if it just won't be possible.  I assumed that if I reduced the size of the character files by deleting some tombstones, this would help, but presumably it won't, as it appears it's the number of files, not the size.

I have started a new Pleasantview ready for the expansion, but am very reluctant to give the old one up as I'm approaching 4th generation on the original families now.  For a while I've been trying to reduce the size by only allowing one child per family, but obviously that doesn't alter the number of files, only the number of playable Sims, so maybe it's a pointless exercise.

Is this going to be extended to other NPC's?  I had a llama mascot die in a dorm recently and the game immediately spawned another one, even though there were still 2 left in play.  I think I read somewhere that mascots are allocated to certain areas, which would explain it.  Is this actually the case?  Seems stupid to create extra characters when I am trying to reduce the number of character files.