rdoc non-determinism: module includes can be added once or twice to generated .ri

We're observing a sporadically occurring non-determinism in generated .ri files:

[ak@localhost ruby-repro]$ ri --dump=./oe-reproducible-20211214-gj4vndgo/packages-excluded/reproducibleA/tmp/deploy/rpm/core2_64/usr/share/ri/3.0.0/system/Gem/DefaultUserInteraction/cdesc-DefaultUserInteraction.ri
[module Gem::DefaultUserInteraction: 
  includes:
  #<RDoc::Include:0x384 Gem::DefaultUserInteraction.rdoc::include Gem::Text>
  
  constants:

....

[ak@localhost ruby-repro]$ ri --dump=./oe-reproducible-20211214-gj4vndgo/packages-excluded/reproducibleB/tmp/deploy/rpm/core2_64/usr/share/ri/3.0.0/system/Gem/DefaultUserInteraction/cdesc-DefaultUserInteraction.ri
[module Gem::DefaultUserInteraction: 
  includes:
  #<RDoc::Include:0x384 Gem::DefaultUserInteraction.rdoc::include Gem::Text>,
  #<RDoc::Include:0x398 Gem::DefaultUserInteraction.rdoc::include Gem::Text>
  
  constants:

This occurs in random .ri files, but the pattern seems same: the difference is always in the includes, and how many times a specific one gets listed.
issue-18456-attachments.zip

Start with the attached reproducer and compare the generated .ri files using ri --dump, focusing on the includes section. Trace why the same module include appears once or twice, then verify that repeated generation produces identical include lists in the affected .ri files.