Breeding two dogs from the same litter with different dads?

I have two dogs from the same litter that CLEARLY have two different dads.  It was clear from the litter which puppies had one dad and which had the other.  I kept one male and all the other dogs found new homes including the mother.  However one of the females came back.  Do I need to have her fixed or since they have two different dad's is it okay if they breed?

Hello - and thank you for this interesting question.  Let's assume for the sake of argument that you are correct and the puppies do have two different Dads (I don't beleive that you can know this, however blatant it may seem from the phenotype (appearance) of the pups, because inheritence isn't so predictable).  But even if we do assume that to be true, then the pups would still share 50% of their DNA so most vets would strongly reccommend against breeding from them.