It's not a problem unless there is a problem with the dog that shows up more than once. Linebreeding can be a very useful tool to increase consistency in a litter. I have a pup that is from a half brother/sister breeding and another that has Hillhavens Hustler show up 7 times in 5 generations. It doesn't mean you will have 2 headed puppies born, but it does increase the chance of both good and bad traits showing up. That is one reason health testing is important. If it's a lab pup, I would make sure both parents have had all the tests done. That would include hips, eyes, cnm, pra and eic.