Remove duplicated elements in CorsConfiguration#combine()

Issue: SPR-14792

spring-web/src/main/java/org/springframework/web/cors/CorsConfiguration.java

@@ -18,7 +18,9 @@ package org.springframework.web.cors;
 
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Set;
 
 import org.springframework.http.HttpMethod;
 import org.springframework.util.CollectionUtils;
@@ -123,12 +125,11 @@ public class CorsConfiguration {
 		if (source == null || source.contains(ALL)) {
 			return other;
 		}
-		List<String> combined = new ArrayList<>(source);
+		Set<String> combined = new LinkedHashSet<>(source);
 		combined.addAll(other);
-		return combined;
+		return new ArrayList<>(combined);
 	}
 
-
 	/**
 	 * Set the origins to allow, e.g. {@code "http://domain1.com"}.
 	 * <p>The special value {@code "*"} allows all domains.

spring-web/src/test/java/org/springframework/web/cors/CorsConfigurationTests.java

@@ -131,6 +131,29 @@ public class CorsConfigurationTests {
 		assertEquals(Arrays.asList(HttpMethod.PUT.name()), combinedConfig.getAllowedMethods());
 	}
 
+	@Test  // SPR-14792
+	public void combineWithDuplicatedElements() {
+		CorsConfiguration config = new CorsConfiguration();
+		config.addAllowedOrigin("http://domain1.com");
+		config.addAllowedOrigin("http://domain2.com");
+		config.addAllowedHeader("header1");
+		config.addAllowedHeader("header2");
+		config.addExposedHeader("header3");
+		config.addExposedHeader("header4");
+		config.addAllowedMethod(HttpMethod.GET.name());
+		config.addAllowedMethod(HttpMethod.PUT.name());
+		CorsConfiguration other = new CorsConfiguration();
+		other.addAllowedOrigin("http://domain1.com");
+		other.addAllowedHeader("header1");
+		other.addExposedHeader("header3");
+		other.addAllowedMethod(HttpMethod.GET.name());
+		CorsConfiguration combinedConfig = config.combine(other);
+		assertEquals(Arrays.asList("http://domain1.com", "http://domain2.com"), combinedConfig.getAllowedOrigins());
+		assertEquals(Arrays.asList("header1", "header2"), combinedConfig.getAllowedHeaders());
+		assertEquals(Arrays.asList("header3", "header4"), combinedConfig.getExposedHeaders());
+		assertEquals(Arrays.asList(HttpMethod.GET.name(), HttpMethod.PUT.name()), combinedConfig.getAllowedMethods());
+	}
+
 	@Test
 	public void combine() {
 		CorsConfiguration config = new CorsConfiguration();