diff --git a/ktor-client/ktor-client-plugins/ktor-client-content-negotiation/common/src/io/ktor/client/plugins/contentnegotiation/ContentNegotiation.kt b/ktor-client/ktor-client-plugins/ktor-client-content-negotiation/common/src/io/ktor/client/plugins/contentnegotiation/ContentNegotiation.kt
index c2f7efb5d76..ceb4a80d0dd 100644
--- a/ktor-client/ktor-client-plugins/ktor-client-content-negotiation/common/src/io/ktor/client/plugins/contentnegotiation/ContentNegotiation.kt
+++ b/ktor-client/ktor-client-plugins/ktor-client-content-negotiation/common/src/io/ktor/client/plugins/contentnegotiation/ContentNegotiation.kt
@@ -140,6 +140,8 @@ public class ContentNegotiation internal constructor(
     internal suspend fun convertRequest(request: HttpRequestBuilder, body: Any): Any? {
         registrations.forEach {
             LOGGER.trace("Adding Accept=${it.contentTypeToSend.contentType} header for ${request.url}")
+
+            if (request.headers.contains(HttpHeaders.Accept, it.contentTypeToSend.toString())) return@forEach
             request.accept(it.contentTypeToSend)
         }
 
diff --git a/ktor-client/ktor-client-plugins/ktor-client-content-negotiation/ktor-client-content-negotiation-tests/jvm/src/io/ktor/client/plugins/contentnegotiation/tests/JsonContentNegotiationTest.kt b/ktor-client/ktor-client-plugins/ktor-client-content-negotiation/ktor-client-content-negotiation-tests/jvm/src/io/ktor/client/plugins/contentnegotiation/tests/JsonContentNegotiationTest.kt
index abf2dbf448c..91b0cacbdee 100644
--- a/ktor-client/ktor-client-plugins/ktor-client-content-negotiation/ktor-client-content-negotiation-tests/jvm/src/io/ktor/client/plugins/contentnegotiation/tests/JsonContentNegotiationTest.kt
+++ b/ktor-client/ktor-client-plugins/ktor-client-content-negotiation/ktor-client-content-negotiation-tests/jvm/src/io/ktor/client/plugins/contentnegotiation/tests/JsonContentNegotiationTest.kt
@@ -280,6 +280,25 @@ abstract class JsonContentNegotiationTest(val converter: ContentConverter) {
         }
     }
 
+    @Test
+    fun testNoDuplicatedHeaders() = testApplication {
+        install(ContentNegotiation) {
+            register(ContentType.Application.Json, converter)
+        }
+
+        createClient {
+            install(io.ktor.client.plugins.contentnegotiation.ContentNegotiation) {
+                register(ContentType.Application.Json, converter)
+            }
+        }.get {
+            header(HttpHeaders.Accept, "application/json")
+        }.let { response ->
+            response.request.headers.forEach { _, values ->
+                assertEquals(1, values.size)
+            }
+        }
+    }
+
     @Test
     open fun testRespondNestedSealedWithTypeInfoAny() = testApplication {
         install(ContentNegotiation) {