refactor: #7 연차 승인/반려 시 검증 로직 구현

src/main/java/com/example/kdtbe5_miniproject/admin/AdminController.java

@@ -1,14 +1,9 @@
 package com.example.kdtbe5_miniproject.admin;
 
-import com.example.kdtbe5_miniproject._core.security.CustomUserDetails;
 import com.example.kdtbe5_miniproject._core.util.ApiUtils;
-import com.example.kdtbe5_miniproject.dayoff.DayOff;
-import com.example.kdtbe5_miniproject.duty.Duty;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.http.ResponseEntity;
-import org.springframework.security.core.annotation.AuthenticationPrincipal;
-import org.springframework.stereotype.Controller;
 import org.springframework.web.bind.annotation.*;
 
 import java.util.HashMap;
@@ -44,6 +39,9 @@ public ResponseEntity<?> userDetails(@PathVariable Long userId) {
 
     @PutMapping("/status/dayoff/{dayOffId}")
     public ResponseEntity<?> dayOffModify(@PathVariable Long dayOffId, @RequestBody AdminRequest.TreatDayOffDTO request) {
+        if (adminService.findWaitingDayOffList(dayOffId).get(0) != dayOffId) {
+            throw new RuntimeException("이미 처리된 요청이거나, 해당 사용자의 이전 요청을 먼저 처리해주세요.");
+        }
         adminService.modifyDayOffStatus(dayOffId, request);
 
         return ResponseEntity.ok().body(ApiUtils.success(request.getStatus() + "되었습니다."));