Avoid duplicate Content-Type in MockHttpServletRequest

Fixes gh-34913

spring-test/src/main/java/org/springframework/mock/web/MockHttpServletRequest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -1064,8 +1064,7 @@ public class MockHttpServletRequest implements HttpServletRequest {
 	 * @see #getDateHeader
 	 */
 	public void addHeader(String name, Object value) {
-		if (HttpHeaders.CONTENT_TYPE.equalsIgnoreCase(name) &&
+		if (HttpHeaders.CONTENT_TYPE.equalsIgnoreCase(name)) {
-				!this.headers.containsKey(HttpHeaders.CONTENT_TYPE)) {
 			setContentType(value.toString());
 		}
 		else if (HttpHeaders.ACCEPT_LANGUAGE.equalsIgnoreCase(name) &&

spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -226,6 +226,15 @@ class MockHttpServletRequestTests {
 		assertThat(request.getCharacterEncoding()).isEqualTo("UTF-8");
 	}
 
+	@Test
+	void contentTypeMultipleCalls() {
+		String contentType = "text/html";
+		request.addHeader(HttpHeaders.CONTENT_TYPE, "text/plain");
+		request.addHeader(HttpHeaders.CONTENT_TYPE, contentType);
+		assertThat(request.getContentType()).isEqualTo(contentType);
+		assertThat(request.getHeader(HttpHeaders.CONTENT_TYPE)).isEqualTo(contentType);
+	}
+
 	@Test  // SPR-12677
 	void setContentTypeHeaderWithMoreComplexCharsetSyntax() {
 		String contentType = "test/plain;charset=\"utf-8\";foo=\"charset=bar\";foocharset=bar;foo=bar";

spring-test/src/test/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilderTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -471,12 +471,12 @@ class MockHttpServletRequestBuilderTests {
 		assertThat(request.getContentType()).isEqualTo("yaml");
 	}
 
-	@Test  // SPR-11308
+	@Test
 	void contentTypeViaMultipleHeaderValues() {
-		this.builder.header("Content-Type", MediaType.TEXT_HTML_VALUE, MediaType.ALL_VALUE);
+		this.builder.header("Content-Type", MediaType.TEXT_HTML_VALUE, MediaType.TEXT_PLAIN_VALUE);
 		MockHttpServletRequest request = this.builder.buildRequest(this.servletContext);
 
-		assertThat(request.getContentType()).isEqualTo("text/html");
+		assertThat(request.getContentType()).isEqualTo("text/plain");
 	}
 
 	@Test

spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletRequest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -1063,8 +1063,7 @@ public class MockHttpServletRequest implements HttpServletRequest {
 	 * @see #getDateHeader
 	 */
 	public void addHeader(String name, Object value) {
-		if (HttpHeaders.CONTENT_TYPE.equalsIgnoreCase(name) &&
+		if (HttpHeaders.CONTENT_TYPE.equalsIgnoreCase(name)) {
-				!this.headers.containsKey(HttpHeaders.CONTENT_TYPE)) {
 			setContentType(value.toString());
 		}
 		else if (HttpHeaders.ACCEPT_LANGUAGE.equalsIgnoreCase(name) &&