diff --git a/doorip-api/src/main/java/org/doorip/user/service/UserService.java b/doorip-api/src/main/java/org/doorip/user/service/UserService.java
index 0fcbee8..ec242a5 100644
--- a/doorip-api/src/main/java/org/doorip/user/service/UserService.java
+++ b/doorip-api/src/main/java/org/doorip/user/service/UserService.java
@@ -4,6 +4,7 @@
 import org.doorip.auth.jwt.JwtProvider;
 import org.doorip.auth.jwt.JwtValidator;
 import org.doorip.auth.jwt.Token;
+import org.doorip.exception.ConflictException;
 import org.doorip.exception.EntityNotFoundException;
 import org.doorip.exception.UnauthorizedException;
 import org.doorip.message.ErrorMessage;
@@ -49,6 +50,7 @@ public UserResponse signIn(String token, UserSignInRequest request) {
     public UserResponse signUp(String token, UserSignUpRequest request) {
         Platform enumPlatform = getEnumPlatformFromStringPlatform(request.platform());
         String platformId = getPlatformId(token, enumPlatform);
+        validateDuplicateUser(enumPlatform, platformId);
         User savedUser = saveUser(request, platformId, enumPlatform);
         Token issueToken = jwtProvider.issueToken(savedUser.getId());
         updateRefreshToken(issueToken.refreshToken(), savedUser);
@@ -83,6 +85,12 @@ private String getPlatformId(String token, Platform platform) {
         return kakaoOAuthProvider.getKakaoPlatformId(token);
     }
 
+    private void validateDuplicateUser(Platform platform, String platformId) {
+        if (userRepository.existsUserByPlatformAndPlatformId(platform, platformId)) {
+            throw new ConflictException(ErrorMessage.DUPLICATE_USER);
+        }
+    }
+
     private User getUser(Platform platform, String platformId) {
         return userRepository.findUserByPlatformAndPlatformId(platform, platformId)
                 .orElseThrow(() -> new EntityNotFoundException(ErrorMessage.USER_NOT_FOUND));
diff --git a/doorip-common/src/main/java/org/doorip/message/ErrorMessage.java b/doorip-common/src/main/java/org/doorip/message/ErrorMessage.java
index a4e6488..997a603 100644
--- a/doorip-common/src/main/java/org/doorip/message/ErrorMessage.java
+++ b/doorip-common/src/main/java/org/doorip/message/ErrorMessage.java
@@ -58,6 +58,7 @@ public enum ErrorMessage {
      * 409 Conflict
      */
     CONFLICT(HttpStatus.CONFLICT, "e4090", "이미 존재하는 리소스입니다."),
+    DUPLICATE_USER(HttpStatus.CONFLICT, "e4091", "이미 존재하는 회원입니다."),
 
     /**
      * 500 Internal Server Error
diff --git a/doorip-domain/src/main/java/org/doorip/user/repository/UserRepository.java b/doorip-domain/src/main/java/org/doorip/user/repository/UserRepository.java
index 539531e..42be847 100644
--- a/doorip-domain/src/main/java/org/doorip/user/repository/UserRepository.java
+++ b/doorip-domain/src/main/java/org/doorip/user/repository/UserRepository.java
@@ -8,4 +8,6 @@
 
 public interface UserRepository extends JpaRepository<User, Long> {
     Optional<User> findUserByPlatformAndPlatformId(Platform platform, String platformId);
+
+    boolean existsUserByPlatformAndPlatformId(Platform platform, String platformId);
 }