Fix bug with batched gitaly ref deletion duplicates

What does this MR do and why?

This is a %16.4 backport of !134323 (merged)

If there are duplicates sent in a single Git command we get an error. This de-duplication works around this. We could also consider de-duplicating in the queue but this is tricky to do without race conditions and would also require a migration. Without the uniq fix this new test fails with:

  1) BatchedGitRefUpdates::ProjectCleanupService#execute handles duplicates
     Failure/Error: raise Gitlab::Git::CommandError, e

     Gitlab::Git::CommandError:
       13:unable to prepare: reference has been updated multiple times within a transaction.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

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