diff --git a/botalka/src/main/kotlin/ru/vityaman/lms/botalka/core/external/yandex/YandexUser.kt b/botalka/src/main/kotlin/ru/vityaman/lms/botalka/core/external/yandex/YandexUser.kt
index 94114ce..56a6849 100644
--- a/botalka/src/main/kotlin/ru/vityaman/lms/botalka/core/external/yandex/YandexUser.kt
+++ b/botalka/src/main/kotlin/ru/vityaman/lms/botalka/core/external/yandex/YandexUser.kt
@@ -5,8 +5,12 @@ data class YandexUser(
     val login: Login,
 ) {
     @JvmInline
-    value class Id(val number: Int)
+    value class Id(val number: Int) {
+        override fun toString(): String = number.toString()
+    }
 
     @JvmInline
-    value class Login(val text: String)
+    value class Login(val text: String) {
+        override fun toString(): String = text
+    }
 }
diff --git a/botalka/src/main/kotlin/ru/vityaman/lms/botalka/storage/jooq/JooqYandexAuthStorage.kt b/botalka/src/main/kotlin/ru/vityaman/lms/botalka/storage/jooq/JooqYandexAuthStorage.kt
index 365fc2a..f3735e4 100644
--- a/botalka/src/main/kotlin/ru/vityaman/lms/botalka/storage/jooq/JooqYandexAuthStorage.kt
+++ b/botalka/src/main/kotlin/ru/vityaman/lms/botalka/storage/jooq/JooqYandexAuthStorage.kt
@@ -1,14 +1,16 @@
 package ru.vityaman.lms.botalka.storage.jooq
 
+import ru.vityaman.lms.botalka.core.exception.InvalidValueException
 import ru.vityaman.lms.botalka.core.external.yandex.YandexUser
 import ru.vityaman.lms.botalka.core.model.User
 import ru.vityaman.lms.botalka.core.storage.YandexAuthStorage
+import ru.vityaman.lms.botalka.storage.jooq.exception.UniqueViolationException
 import ru.vityaman.lms.botalka.storage.jooq.tables.references.AUTH_YANDEX
 
 class JooqYandexAuthStorage(
     private val database: JooqDatabase,
 ) : YandexAuthStorage {
-    override suspend fun create(userId: User.Id, yandexUser: YandexUser) =
+    override suspend fun create(userId: User.Id, yandexUser: YandexUser) = try {
         database.only {
             insertInto(
                 AUTH_YANDEX,
@@ -21,6 +23,15 @@ class JooqYandexAuthStorage(
                 yandexUser.login.text,
             )
         }.let { }
+    } catch (e: UniqueViolationException) {
+        throw InvalidValueException(
+            buildString {
+                append("yandex user '${yandexUser.login}' ")
+                append("(id '${yandexUser.id}') already connected")
+            },
+            e,
+        )
+    }
 
     override suspend fun corresponding(id: YandexUser.Id): User.Id? =
         database.maybe {
diff --git a/botalka/src/test/kotlin/ru/vityaman/lms/botalka/app/spring/api/http/endpoint/AuthApiTest.kt b/botalka/src/test/kotlin/ru/vityaman/lms/botalka/app/spring/api/http/endpoint/AuthApiTest.kt
index 4c38040..2a25c8a 100644
--- a/botalka/src/test/kotlin/ru/vityaman/lms/botalka/app/spring/api/http/endpoint/AuthApiTest.kt
+++ b/botalka/src/test/kotlin/ru/vityaman/lms/botalka/app/spring/api/http/endpoint/AuthApiTest.kt
@@ -3,6 +3,8 @@ package ru.vityaman.lms.botalka.app.spring.api.http.endpoint
 import kotlinx.coroutines.reactor.awaitSingle
 import kotlinx.coroutines.runBlocking
 import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertThrows
+import org.springframework.web.reactive.function.client.WebClientResponseException
 import ru.vityaman.lms.botalka.app.spring.BotalkaTestSuite
 import ru.vityaman.lms.botalka.app.spring.api.http.client.Api
 import ru.vityaman.lms.botalka.app.spring.api.http.client.Auth2ViaCodeMessage
@@ -32,4 +34,31 @@ class AuthApiTest : BotalkaTestSuite() {
             ),
         ).awaitSingle()
     }
+
+    @Test
+    fun duplicateRegistration(): Unit = runBlocking {
+        val api = Api.ofNewbie()
+
+        api.user.postUser(
+            PostUserRequestMessage(
+                user = UserDraftMessage("vanya"),
+                credential = Auth2ViaCodeMessage(
+                    kind = CredentialKindMessage.yandex_code,
+                    code = 654_321,
+                ),
+            ),
+        ).awaitSingle()
+
+        assertThrows<WebClientResponseException.BadRequest> {
+            api.user.postUser(
+                PostUserRequestMessage(
+                    user = UserDraftMessage("vitya"),
+                    credential = Auth2ViaCodeMessage(
+                        kind = CredentialKindMessage.yandex_code,
+                        code = 654_321,
+                    ),
+                ),
+            ).awaitSingle()
+        }
+    }
 }