[WIP] Add tests for removing duplicate keys in scls format

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add tests for removing duplicate keys</issue_title>
<issue_description>We have a requirement that we have only one value with a given key in each namespace, but currently we do not have tests for that and likely do not ensure that. Would be nice to add the tests for that and fix an implementation if it's not a case.

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The first action in this issue will be to introduce set of tests that checks that values with the duplicated keys are not stored in the scls format. It seems that we need to either use MultiNamespace tests or introduce a new module spec.</issue_description>

<agent_instructions>1. Introduce a new set of tests
2. It's ok that new test will fail to work (not ok if it fails to compile)
</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #84

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