Fix: add Id Uniqueness Validation

[dydghk456]

PR Title: 중복 ID 검사 로직 추가

Related Issue(s):

PR Description:

id 중복 검사 기능 추가

Changes Included:

• Added new feature(s)
• Fixed identified bug(s)
• Updated relevant documentation

Screenshots (if UI changes were made):

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frontend-related changes)

Notes for Reviewer:

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reviewer.

Reviewer Checklist:

• Code is written in clean, maintainable, and idiomatic form.
• Automated test coverage is adequate.
• All existing tests pass.
• Manual testing has been performed to ensure the PR works as
  expected.
• Code review comments have been addressed or clarified.

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Additional Comments:

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review process.

[Jack-Chagarr]

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