Apology - didn't tell the story right.
I have been told that a GSD lost her litter and the vet told them that it was due to her being mated twice.

The story goes -  there were 2 litters. One litter born about 3 days before the other litter. The owners were not aware of the second litter. All were stillborn and the vet told them that it was due to her being mated twice.

After searching the Internet I found - "Superfetation (also spelt superfoetation) is the formation of a fetus while another fetus is already present in the uterus. When there are two separate instances of fertilisation during the same cycle, rather than different cycles, it is known as superfecundation.
It is claimed to be common in some species of animals and it can only occur where there are two uteri.
Or - The simultaneous development of several litters/broods within the ovary where they are nourished; enabled by the entrance and storage of sperm in the ovary."

I had never heard of this and wondered if anyone else had come across this problem?

There are replies on the other thread - Admin will remove this duplicate thread to avoid confusion.