Yes they can actually. Dogs produce multiple eggs(ovums) and one sperm can only fertilize an egg one time. If a dog was bred to two separate dogs within the time of being receptive(in heat), sperm from dog can fertilize one egg, while sperm from the other dog can fertilize another egg.
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Yes, this is possible but not without consequences. This is inbreeding and the resulting puppies will have little genetic diversity, weakened immune systems, deformities and general lack of good health. For this reason, breeding a father with his daughter is unadviseable.
They can but it is very rare, it only happens if the female mates with more than one male while she is on heat.