De-duplicate entries during validation

[beechnut]

During the validation process, scan the glossary for duplicates. The error message should read something like:

I found 3 entries for "ATO".

How can I know which one to use?

Please delete or combine until there is only one entry for "ATO".

[beechnut]

Discovered that finding duplicate keys in YAML is a ten-year-old issue with a two-year-old PR.

This will take a little extra work.