diff --git a/spring-core/src/main/java/org/springframework/util/MimeTypeUtils.java b/spring-core/src/main/java/org/springframework/util/MimeTypeUtils.java
index 24add7f4f3d..55f76d94820 100644
--- a/spring-core/src/main/java/org/springframework/util/MimeTypeUtils.java
+++ b/spring-core/src/main/java/org/springframework/util/MimeTypeUtils.java
@@ -252,7 +252,9 @@ public abstract class MimeTypeUtils {
 				if (eqIndex >= 0) {
 					String attribute = parameter.substring(0, eqIndex).trim();
 					String value = parameter.substring(eqIndex + 1).trim();
-					parameters.put(attribute, value);
+					if (parameters.put(attribute, value) != null) {
+						throw new InvalidMimeTypeException(mimeType, "duplicate parameter '" + parameter + "'");
+					}
 				}
 			}
 			index = nextIndex;
diff --git a/spring-core/src/test/java/org/springframework/util/MimeTypeTests.java b/spring-core/src/test/java/org/springframework/util/MimeTypeTests.java
index 6346b8ec540..cffa8d31e5f 100644
--- a/spring-core/src/test/java/org/springframework/util/MimeTypeTests.java
+++ b/spring-core/src/test/java/org/springframework/util/MimeTypeTests.java
@@ -32,6 +32,7 @@ import static java.util.Collections.singletonMap;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 /**
  * Tests for {@link MimeType}.
@@ -116,6 +117,13 @@ class MimeTypeTests {
 		assertThat(mimeType.getParameter("twelve")).isEqualTo("\"1\\\"2\"");
 	}
 
+	@Test
+	void rejectsDuplicateParameter() {
+		String s = "text/plain;dupe=\"1\";dupe=\"2\"";
+		assertThatThrownBy(() -> MimeType.valueOf(s)).isInstanceOf(InvalidMimeTypeException.class)
+				.hasMessageContaining("Invalid mime type \"text/plain;dupe=\"1\";dupe=\"2\"\": duplicate parameter 'dupe=\"2\"'");
+	}
+
 	@Test
 	void withConversionService() {
 		ConversionService conversionService = new DefaultConversionService();
diff --git a/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java b/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java
index e416e3fb44d..5184b5bf8cf 100644
--- a/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java
+++ b/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java
@@ -237,7 +237,7 @@ class MockHttpServletRequestTests {
 
 	@Test  // SPR-12677
 	void setContentTypeHeaderWithMoreComplexCharsetSyntax() {
-		String contentType = "test/plain;charset=\"utf-8\";foo=\"charset=bar\";foocharset=bar;foo=bar";
+		String contentType = "test/plain;charset=\"utf-8\";foo=\"charset=bar\";foocharset=bar;foobar=bar";
 		request.addHeader(HttpHeaders.CONTENT_TYPE, contentType);
 		assertThat(request.getContentType()).isEqualTo(contentType);
 		assertThat(request.getHeader(HttpHeaders.CONTENT_TYPE)).isEqualTo(contentType);
diff --git a/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java b/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java
index baf67c88f69..ef0df3a1719 100644
--- a/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java
+++ b/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java
@@ -116,7 +116,7 @@ class MockHttpServletResponseTests {
 
 		@Test // SPR-12677
 		void shouldSetEncodingWithComplexContentTypeSyntax() {
-			String contentType = "test/plain;charset=\"utf-8\";foo=\"charset=bar\";foocharset=bar;foo=bar";
+			String contentType = "test/plain;charset=\"utf-8\";foo=\"charset=bar\";foocharset=bar;foobar=bar";
 			response.setHeader(HttpHeaders.CONTENT_TYPE, contentType);
 			assertThat(response.getContentType()).isEqualTo(contentType);
 			assertThat(response.getHeader(HttpHeaders.CONTENT_TYPE)).isEqualTo(contentType);