Dear Maintainer,

I'm an occasional volunteer contributor to the Reproducible Builds[1] project,
and noticed recently that the bind9-doc package failed[2] an automated Debian
package build test.

From inspecting the differences in the generated documentation, it appears that
some tag annotations (example[3]) in the source documentation are de-duplicated
in a non-deterministic manner during Sphinx documentation builds.

The cause is that the de-duplication is performed using a non-order-preserving
Python set[4] object, meaning that the resulting output documentation package
can vary at build-time.

Please find attached a patch that implements order-preserving de-duplication
of these tags.  I've confirmed that the package compiles with it applied, and
believe that it should produce deterministic tag output in the built package.

Regards,
James

[1] - https://reproducible-builds.org/

[2] - https://tests.reproducible-builds.org/debian/rb-pkg/bookworm/amd64/diffoscope-results/bind9.html

[3] - https://sources.debian.org/src/bind9/1%3A9.18.19-1~deb12u1/doc/arm/reference.rst/#L5648

[4] - https://docs.python.org/3/library/stdtypes.html#set-types-set-frozenset