So sorry for your loss of Chewy.
Duke is sooo adorable; I'm sure waiting for him will be tough! We did multiple puppies at once and while it is not easy I think if I had been an experienced boxer owner it would have been easier. You have had multiple boxers that have thrived so I am sure you would be able to raise two puppies and give each his own time and energy to allow him to be his best boxer. I think just knowing about littermate syndrome and what to do to avoid it gives you an additional advantage. Your girl may not be so happy with two new little crazies,
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Good luck with Duke and keep us posted