For reasons unknown to me, installing databases/postgresql13-contrib tries to install databases/postgresql12-client (Version 12, not 13!) due to line
USES= bison pgsql:12
in the Makefile. This ends up in a conflict when databases/postgresql13-client is already installed (which usually does). I can only assume this is a typo when derived postgresql 13 port from its progenitor 12.
Thanks for reporting! This was fixed in 249888.
*** This bug has been marked as a duplicate of bug 249888 ***