ISSUE-7755: Prevent adding duplicate graders on CSV upload

[Naragod]

Proposed Changes

When uploading grader-group mappings (via Graders -> Upload) without the Remove existing grader assignments option selected, a grader can be assigned to the same group multiple times. We wish to prevent this from happening by default.

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Associated documentation repository pull request (if applicable)

Type of Change

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Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)
🐛 Bug fix (non-breaking change that fixes an issue)	X
🎨 User interface change (change to user interface; provide screenshots)
♻️ Refactoring (internal change to codebase, without changing functionality)
🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)

Checklist

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Before opening your pull request:

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  • Check that all changes are relevant to the purpose of this pull request, as described above.
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[coveralls]

Pull Request Test Coverage Report for Build 21273694870

Details

• 36 of 36 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 91.648%

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Totals
Change from base Build 21224303427:	0.006%
Covered Lines:	44331
Relevant Lines:	47563

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💛 - Coveralls

[donny-wong]

thanks @Naragod , I left 1 comment.