Add warning on double suppression

[timotheeguerin]

When the same code is suppressed twice on the same node we should add a warning

model Test {
  #suppress "deprecated" "Msg"
  #suppress "deprecated" "Msg"
  prop: Dep;
}

with or without message

[github-actions]

🎯 Issue Triage

Feature request to have the compiler emit a warning when the same diagnostic code is suppressed more than once on the same node.

Category: Feature Request
Area: compiler:core

🔍 Analysis

• The #suppress directive is processed by the TypeSpec compiler core.
• Duplicate suppressions on the same node are a no-op but can indicate typos, copy-paste errors, or leftover code that the developer should clean up.
• A diagnostic warning (similar to an "unused suppression" warning) would improve code hygiene.
• The check would need to track per-node suppress directives and flag duplicates during compilation.

📝 Suggested next steps

• Add a new diagnostic (e.g., suppress-duplicate) in the compiler's diagnostics registry.
• During suppress directive processing, detect when the same code is applied to the same node more than once and emit the new warning.
• Add tests covering: exact duplicate (same code + same message), same code with different messages, and no duplicate (should not warn).

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