diff --git a/Domain-Module/src/main/kotlin/com/bamyanggang/domainmodule/domain/user/exception/UserException.kt b/Domain-Module/src/main/kotlin/com/bamyanggang/domainmodule/domain/user/exception/UserException.kt
new file mode 100644
index 00000000..05cf705d
--- /dev/null
+++ b/Domain-Module/src/main/kotlin/com/bamyanggang/domainmodule/domain/user/exception/UserException.kt
@@ -0,0 +1,16 @@
+package com.bamyanggang.domainmodule.domain.user.exception
+
+import com.bamyanggang.commonmodule.exception.CustomException
+
+sealed class UserException(
+    errorCode: Int,
+    message: String,
+) : CustomException(CODE_PREFIX, errorCode, message) {
+
+    class DuplicatedUser(message: String = "이미 존재하는 유저입니다.") :
+        UserException(errorCode = 1, message = message)
+
+    companion object {
+        const val CODE_PREFIX = "USER"
+    }
+}
diff --git a/Domain-Module/src/main/kotlin/com/bamyanggang/domainmodule/domain/user/repository/UserRepository.kt b/Domain-Module/src/main/kotlin/com/bamyanggang/domainmodule/domain/user/repository/UserRepository.kt
index 27a74dc5..09e19b80 100644
--- a/Domain-Module/src/main/kotlin/com/bamyanggang/domainmodule/domain/user/repository/UserRepository.kt
+++ b/Domain-Module/src/main/kotlin/com/bamyanggang/domainmodule/domain/user/repository/UserRepository.kt
@@ -11,4 +11,6 @@ interface UserRepository {
 
  fun findById(userId: UUID): User
 
-}
\ No newline at end of file
+ fun existsBySocialId(socialId: String): Boolean
+
+}
diff --git a/Domain-Module/src/main/kotlin/com/bamyanggang/domainmodule/domain/user/service/UserAppender.kt b/Domain-Module/src/main/kotlin/com/bamyanggang/domainmodule/domain/user/service/UserAppender.kt
index aca24335..cbd33b14 100644
--- a/Domain-Module/src/main/kotlin/com/bamyanggang/domainmodule/domain/user/service/UserAppender.kt
+++ b/Domain-Module/src/main/kotlin/com/bamyanggang/domainmodule/domain/user/service/UserAppender.kt
@@ -2,6 +2,7 @@ package com.bamyanggang.domainmodule.domain.user.service
 
 import com.bamyanggang.domainmodule.domain.user.aggregate.User
 import com.bamyanggang.domainmodule.domain.user.enums.SocialLoginProvider
+import com.bamyanggang.domainmodule.domain.user.exception.UserException
 import com.bamyanggang.domainmodule.domain.user.repository.UserRepository
 
 class UserAppender(
@@ -18,6 +19,7 @@ class UserAppender(
         goal: String?,
         dream: String?
     ): User {
+        if(userRepository.existsBySocialId(socialId)) {throw UserException.DuplicatedUser()}
         return User.create(
             socialId = socialId,
             profileImgUrl = profileImgUrl,
diff --git a/Domain-Module/src/test/kotlin/com/bamyanggang/domainmodule/domain/user/service/UserAppenderTest.kt b/Domain-Module/src/test/kotlin/com/bamyanggang/domainmodule/domain/user/service/UserAppenderTest.kt
new file mode 100644
index 00000000..8593224c
--- /dev/null
+++ b/Domain-Module/src/test/kotlin/com/bamyanggang/domainmodule/domain/user/service/UserAppenderTest.kt
@@ -0,0 +1,53 @@
+package com.bamyanggang.domainmodule.domain.user.service
+
+import com.bamyanggang.domainmodule.domain.user.enums.SocialLoginProvider
+import com.bamyanggang.domainmodule.domain.user.exception.UserException
+import com.bamyanggang.domainmodule.domain.user.repository.UserRepository
+import io.kotest.assertions.throwables.shouldThrow
+import io.kotest.core.spec.style.BehaviorSpec
+import io.mockk.every
+import io.mockk.mockk
+import io.mockk.verify
+
+class UserAppenderTest : BehaviorSpec({
+    val mockUserRepository = mockk<UserRepository>(relaxed = true)
+    val userAppender = UserAppender(mockUserRepository)
+    val socialId = "socialId"
+    val profileImgUrl = "profileImgUrl"
+    val provider = SocialLoginProvider.KAKAO
+    val email = "email"
+    val nickName = "nickName"
+    val jobSearchStatus = "jobSearchStatus"
+    val desiredJob = "desiredJob"
+    val goal = "goal"
+    val dream = "dream"
+
+    Given("a social id, profile image url, provider, email, nickname, job search status, desired job, goal, and dream") {
+        every { mockUserRepository.existsBySocialId(socialId) } returns false
+
+        When("appendUser is called") {
+            userAppender.appendUser(socialId, profileImgUrl, provider, email, nickName, jobSearchStatus, desiredJob, goal, dream)
+
+            Then("a new user should be created and saved") {
+                verify { mockUserRepository.existsBySocialId(socialId) }
+                verify { mockUserRepository.save(any()) }
+            }
+        }
+    }
+
+    Given("a social id that already exists") {
+        every { mockUserRepository.existsBySocialId(socialId) } returns true
+
+        When("appendUser is called") {
+            val exception = shouldThrow<UserException.DuplicatedUser> {
+                userAppender.appendUser(socialId, profileImgUrl, provider, email, nickName, jobSearchStatus, desiredJob, goal, dream)
+            }
+
+            Then("a DuplicatedUser exception should be thrown") {
+                verify { mockUserRepository.existsBySocialId(socialId) }
+                assert(exception.message == "이미 존재하는 유저입니다.")
+            }
+        }
+    }
+
+})
diff --git a/Infrastructure-Module/persistence/src/main/java/com/bamyanggang/persistence/user/UserRepositoryImpl.java b/Infrastructure-Module/persistence/src/main/java/com/bamyanggang/persistence/user/UserRepositoryImpl.java
index 497b7812..04d62d13 100644
--- a/Infrastructure-Module/persistence/src/main/java/com/bamyanggang/persistence/user/UserRepositoryImpl.java
+++ b/Infrastructure-Module/persistence/src/main/java/com/bamyanggang/persistence/user/UserRepositoryImpl.java
@@ -32,9 +32,15 @@ public User findBySocialId(String socialId) {
     }
 
     @Override
-    public User findById(@NotNull UUID userId) {
+    public User findById(UUID userId) {
         return userJpaRepository.findByUserId(userId)
                 .map(userMapper::toDomainEntity)
                 .orElseThrow(() -> new PersistenceException.NotFound());
     }
-}
\ No newline at end of file
+
+    @Override
+    public boolean existsBySocialId(String socialId) {
+        return userJpaRepository.existsBySocialId(socialId);
+    }
+
+}
diff --git a/Infrastructure-Module/persistence/src/main/java/com/bamyanggang/persistence/user/jpa/repository/UserJpaRepository.java b/Infrastructure-Module/persistence/src/main/java/com/bamyanggang/persistence/user/jpa/repository/UserJpaRepository.java
index 0b909d46..071badee 100644
--- a/Infrastructure-Module/persistence/src/main/java/com/bamyanggang/persistence/user/jpa/repository/UserJpaRepository.java
+++ b/Infrastructure-Module/persistence/src/main/java/com/bamyanggang/persistence/user/jpa/repository/UserJpaRepository.java
@@ -10,4 +10,6 @@ public interface UserJpaRepository extends JpaRepository<UserJpaEntity, UUID> {
 
     Optional<UserJpaEntity> findByUserId(UUID id);
 
+    boolean existsBySocialId(String socialId);
+
 }