Group Code Owner rules with matching patterns and sections

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When a file pattern is repeated within a section, group the owners together instead of using the "last rule wins" approach. For example

[Documentation]
README.md @test

[DocuMentatION]
README.md @delicia

Currently, this gives us

Documentation
README.md   @delicia  

..but should result in

Documentation
README.md   @test @delicia 

(h/t to @m_gill for this example: !36383 (comment 376571396))

Additionally, this would be a good Code Owners Linter/Verification item to flag as a 💛