I have been told that a GSD has lost her litter and the owners were told by the vet that it was due to be mated twice. I've searched the Internet and come across superfoetation & superfecundation. Can anyone enlighten me on this?

Two or even three matings, on alternate days, are normal. Litter size depends on how many eggs the bitch releases, and at what stage of her cycle they meet the sperm. A single mating will put several million sperm into the tract, but it won't increase the number of eggs released ...

Apology - didn't tell the story right. Apparently, there were 2 litters. One litter born about 3 days before the other litter. All were stillborn and the vet told them that it was due to her being mated twice.

Information from Internet states - 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.

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

It would seem unlikely in the canine which is an animal designed to ahve litters.  The bitch ovulates over a 24 to 48 hour period and the eggs would only be viable and fertilizable for roughly 48 hours so the ages of fetuses could not be so wildly out as to cause a problem as a week before or after due date should give viable offspring.  That is my understanding of canine reproduction from everything I have read on the matter.

In the nastural state a bitch would mate with a male many times over a period of up to a week, though fertilization would only occur over roughtly a 48 hour period..

There are many cases of mixed litters, where puppies within the litter have different sires, from matings over the bitch's fertile period of about 5 days. It doesn't usually cause a problem.

sorry for sounding thick here...but if a bitch was mated every day from day 9-15 for example surely she would only concieve when she if fully ovulating 24 to 48 hrs within that cycle

Yes that is correct, but nature has made the standing time longer (especially before ovulation) to maximise the chances of conception, as sperm can survive quite a long time in the bitches tract, so even if only one mating occurs quite early if enough viable sperm remain then a conception can take place, though chances are that the litter will be small.

I'm glad its not just me who couldn't take this in. But wouldn't it still be a problem if the bitch did have 2 uteri?(think thats the correct plural). And double ovulation I guess you would say is what this is about. Found this - www.maddogsonline.co.uk/breeding/Breeding-report14.htm

Thing is a bitch has only one uterus which has two horns but they are not seperate so I woudl think if there were two episodes of birtha dn all dead that this was abortion/miscariage and not whelping due to some mishap in late pregnancy, failure of the placentas or whatever causing the pups deaths.

I would expect that having failed to absorb the dead fetal tissue the bitch simply had two goes at ridding her body of the dead tissues.

Two matings, 48 hours apart, are generally recommended. The eggs are immature when released and ripen in the oviduct for about 4 days. Sperm can live in the tract for up to 7 days, so the chances of 'mixed' litters from repeat matings is high. However the fertilised eggs don't implant into the the uterine horns (a bitch's uterus comprises two horns, rather than being the single cavity that humans have) until about 21 days post-fertilisation; before that they're floating freely.