When Dennis was a puppy he grew up with our friends dog Ted, a border Collie. Sadly he is no longer with us. Our friend has now taken on her sisters dog who happens to be a border collie also but brown/white and long haired. Whenever we see this dog out Dennis is SO excited! He runs off (which is unusual as he usually just plods!) and with his tail wagging so hard! Even once he’s caught up with the dog he stays so excited and wants to follow him everywhere. He doesn’t seem to register that the dog isn’t Ted. The only thing I can think of is that this other dog smells the same as what Ted did (living in same house/same owner) Does this mean dogs don’t recognise others in appearance? Just by smell?