No. Once you are pregnant the ovulation stops and the uterus is also closed by the mucus plug.
It happens quite often. It is called the vanishing twin syndrom
why would you want to?
No different than if pregnant with one.
You can expect a lot of things when pregnant with twins. You will have more scans, you will be bigger and more uncomfortable, and you will need more preparation of when the special day arrives.
Not necessarily you could be having twins and they don't have to be identical.
While pregnant you shouldn't even try particularly hard. Slimming down just about always mean losing weight, and one should be really careful about attempting to lose weight while pregnant. You need a decent food intake for the baby to grow and develop as it should.
If you ment "Twins" while one of your sims is pregnant go into the cheat menu (ctrl+shift+c) and enter "forceTwins" OR If you want to not cheat It is a matter of chance
Cows typically give birth to one calf at a time when pregnant. Twins are rare but can occur in some cases.
You can't particularly "diet" to lose weight, but you can control the calorie intake to a level that is healthy for your body type and pregnancy. This reduction in calories will overall make you lose weight, especially after you deliver the baby.
Actually this is very possible and it has happened to many women.
No she can not conceive another baby if there is already one in there. The only way that there would be two babies was if they were twins.