
Yes I have had this, usually when you plan a mating, or they come in unexpectedly early.
In 2006 I planned a last litter from my champion bitch to a new import, and then was travelling abroad to mate her daughter. They were due in season about 2 - 3 months apart so theory was one litter would have been reared before the other was born.
In the end they came in season 3 days apart, the older one being late by two months. I assumed one or other might not take, especially as the older one could be a pain to get mated always wanting to play the male.
In the end Jozi stood like a dream while I was away with her daughter Lexi who was mated abroad, no problem (both had missed on previous attempt with an inexperienced male), both ended up with 7 pups (my overall average litter size has been just shy of 6, 4 - 7 most often, with a range of 3 - 9).
For my US trip Inka came in a month early during all that snow, cancelled flights and shut airports in 2010.