318: add tests for duplicate loads and errors

[John-Sharples]

More test, mostly covering how duplicates are handelled.

Pull Request Testing

• Describe testing already performed for these changes:
  
  Tests pass locally

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  
  Review and run tests

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes or No]
  NA

• Do these changes include sufficient testing updates? [Yes or No]
  Yes

• Will this PR result in changes to the test suite? [Yes or No]
  
  If yes, describe the new output and/or changes to the existing output:
  
  Yes - more tests

• Do these changes introduce new SonarQube findings? [Yes or No]
  
  If yes, please describe:

• Please complete this pull request review by [Fill in date].
  

Pull Request Checklist

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• Add any new Python packages to the METplus Components Python Requirements table.
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• Complete the PR definition above.
• Ensure the PR title matches the feature or bugfix branch name.
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  Select: Milestone as the version that will include these changes
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