diff --git a/spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultClientResponseBuilder.java b/spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultClientResponseBuilder.java
index ce5e6c4dba9..6c5473185e0 100644
--- a/spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultClientResponseBuilder.java
+++ b/spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultClientResponseBuilder.java
@@ -18,6 +18,7 @@ package org.springframework.web.reactive.function.client;
 
 import java.net.URI;
 import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.Map;
 import java.util.function.Consumer;
@@ -144,7 +145,7 @@ final class DefaultClientResponseBuilder implements ClientResponse.Builder {
 	@SuppressWarnings({"ConstantConditions", "NullAway"})
 	private HttpHeaders getHeaders() {
 		if (this.headers == null) {
-			this.headers = new HttpHeaders(this.originalResponse.headers().asHttpHeaders());
+			this.headers = HttpHeaders.copyOf(this.originalResponse.headers().asHttpHeaders());
 		}
 		return this.headers;
 	}
@@ -166,7 +167,10 @@ final class DefaultClientResponseBuilder implements ClientResponse.Builder {
 	@SuppressWarnings({"ConstantConditions", "NullAway"})
 	private MultiValueMap<String, ResponseCookie> getCookies() {
 		if (this.cookies == null) {
-			this.cookies = new LinkedMultiValueMap<>(this.originalResponse.cookies());
+			MultiValueMap<String, ResponseCookie> originalCookies = this.originalResponse.cookies();
+			this.cookies = new LinkedMultiValueMap<>(originalCookies.size());
+			originalCookies.forEach(
+					(name, values) -> this.cookies.put(name, new ArrayList<>(values)));
 		}
 		return this.cookies;
 	}
diff --git a/spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultClientResponseBuilderTests.java b/spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultClientResponseBuilderTests.java
index bb1f4056296..6fb195097df 100644
--- a/spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultClientResponseBuilderTests.java
+++ b/spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultClientResponseBuilderTests.java
@@ -83,16 +83,21 @@ class DefaultClientResponseBuilderTests {
 		ClientResponse result = otherResponse.mutate()
 				.statusCode(HttpStatus.BAD_REQUEST)
 				.headers(headers -> headers.set("foo", "baar"))
-				.cookies(cookies -> cookies.set("baz", ResponseCookie.from("baz", "quux").build()))
+				.cookies(cookies -> cookies.add("baz", ResponseCookie.from("baz", "pop").build()))
 				.build();
 
+		assertThat(otherResponse.headers().asHttpHeaders().getFirst("foo")).isEqualTo("bar");
+		assertThat(otherResponse.headers().asHttpHeaders().getFirst("bar")).isEqualTo("baz");
+		assertThat(otherResponse.cookies().get("baz")).hasSize(1);
 
 		assertThat(result.statusCode()).isEqualTo(HttpStatus.BAD_REQUEST);
 		assertThat(result.headers().asHttpHeaders().size()).isEqualTo(3);
 		assertThat(result.headers().asHttpHeaders().getFirst("foo")).isEqualTo("baar");
 		assertThat(result.headers().asHttpHeaders().getFirst("bar")).isEqualTo("baz");
 		assertThat(result.cookies()).hasSize(1);
-		assertThat(result.cookies().getFirst("baz").getValue()).isEqualTo("quux");
+		assertThat(result.cookies().get("baz")).hasSize(2);
+		assertThat(result.cookies().getFirst("baz").getValue()).isEqualTo("qux");
+		assertThat(result.cookies().get("baz").get(1).getValue()).isEqualTo("pop");
 		assertThat(result.logPrefix()).isEqualTo("my-prefix");
 
 		StepVerifier.create(result.bodyToFlux(String.class))