Barking at next doors dog

[BabeRuthless]

Our pup is coming up to six months old and has been with us for four. Our next door neighbours moved in a couple of months ago with a dog I'm guessing is a couple of months old. Our dog barked at it a bit at first but then soon settled down. Now the neighbours have thrown a puppy into the mix which has sent our dog absolutely nuts. They're keeping it in an upstairs bedroom and when it barks our dog goes crazy. Now when anyone is in next doors garden she barks
incessantly to the point where ds has been in tears this afternoon. She doesn't behave like this when we're on walks and has never shown any fear or aggression to any other dog. Any tips on how we can handle this?