I’ve run into this a few times using pkgin. Not sure if it’s a problem with pkgin or with the packages and their dependencies. What seems to work for me is to remove the newer version of the package that it’s complaining about, then retrying the installation. In you case, remove boost-headers-1.67.0nb4, then try the install of libreoffice again.
This may not be the best or even correct solution, especially if you have lots of packages installed. But in my installation I only have a few packages installed and this seems to get me past that hurdle.
