feat: 자동 이슈 생성에 중복 체크 로직 추가

- 같은 제목의 이슈가 이미 존재하면 생성 건너뛰기
- weekly-challenge 라벨이 있는 이슈 검색
- 중복 방지로 안전한 자동화 구현

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Gyuhyeok99

.github/workflows/auto-weekly-issue.yml

@@ -29,7 +29,36 @@ jobs:
           echo "start_date=$START_DATE" >> $GITHUB_OUTPUT
           echo "end_date=$END_DATE" >> $GITHUB_OUTPUT
 
+      - name: Check for duplicate issue
+        id: check
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const weekNumber = '${{ steps.calc.outputs.week_number }}';
+            const startDate = '${{ steps.calc.outputs.start_date }}';
+            const endDate = '${{ steps.calc.outputs.end_date }}';
+            const expectedTitle = `[Week${weekNumber}] ${startDate} ~ ${endDate} 주차 문제`;
+
+            // 열려있는 이슈 중에서 같은 제목이 있는지 확인
+            const { data: issues } = await github.rest.issues.listForRepo({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              state: 'all',
+              labels: 'weekly-challenge'
+            });
+
+            const duplicate = issues.find(issue => issue.title === expectedTitle);
+
+            if (duplicate) {
+              console.log(`⚠️ Issue already exists: ${duplicate.html_url}`);
+              return 'skip';
+            } else {
+              console.log(`✅ No duplicate found. Proceeding to create issue.`);
+              return 'create';
+            }
+
       - name: Create weekly issue
+        if: steps.check.outputs.result == 'create'
         uses: actions/github-script@v7
         with:
           script: |