This is probably the only time Story Progression will come in handy. So turn it on.
The Doppelganger Challenge doesn't take long, it may only take a weekend.
1. Create a new save game and make a single young adult Sim in CAS. You can roll traits if you want.
2. Place a copy in the neighborhood. Don't play just yet.
3. Instead, place another copy of the same Sim on the other side of the neighborhood. Hence, the doppelganger part of the title.
4. Play the first copy for three Sim-days.
5. Go to 'Edit Town' and change the active household to the other copy. Story progression may have made some... additions to the family.
6. Play the copy for three Sim-days and then switch back to the other. The game will soon have your Sims in crazy situations only Story Progression could come up with.
No point system, because people tend to not use them. I may draft one later, though.
Building cheats allowed, but not money cheats.
Before you protest, you ARE allowed to boost and lock motives using 'testingcheatsenabled true'. This means you don't have to expend Simoleons on fridges, tubs or toilets, making housing affordable without money cheats.