It's more likely that the twins were [indicating] different gestational ages, which is pretty common.
Dr Mindy Christiansen, an assistant professor of gynaecology and obstetrics at Johns Hopkins Medical School, added:
There's a very narrow window for fertilisation and implantation to occur.
In this situation there could have been a delayed fertilisation of one of the eggs, followed by a delayed implantation. That's not really two separate pregnancies.
The article also points out that the unlikelihood of sperm existing in the woman's body for ten days to fertilise the second egg, combined with the incredible rarity of conceiving while already pregnant, makes the superfoetation explanation seem unlikely.