Fix SystemEnvironmentPropertyMapper tests and remove duplicate names

See gh-45741

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/SystemEnvironmentPropertyMapper.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.context.properties.source;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Locale;
 import java.util.function.BiPredicate;
@@ -40,10 +41,18 @@ final class SystemEnvironmentPropertyMapper implements PropertyMapper {
 
 	@Override
 	public List<String> map(ConfigurationPropertyName configurationPropertyName) {
-		return List.of(configurationPropertyName.toString(ToStringFormat.SYSTEM_ENVIRONMENT, true),
-				configurationPropertyName.toString(ToStringFormat.LEGACY_SYSTEM_ENVIRONMENT, true),
-				configurationPropertyName.toString(ToStringFormat.SYSTEM_ENVIRONMENT, false),
-				configurationPropertyName.toString(ToStringFormat.LEGACY_SYSTEM_ENVIRONMENT, false));
+		List<String> mapped = new ArrayList<>(4);
+		addIfMissing(mapped, configurationPropertyName.toString(ToStringFormat.SYSTEM_ENVIRONMENT, true));
+		addIfMissing(mapped, configurationPropertyName.toString(ToStringFormat.LEGACY_SYSTEM_ENVIRONMENT, true));
+		addIfMissing(mapped, configurationPropertyName.toString(ToStringFormat.SYSTEM_ENVIRONMENT, false));
+		addIfMissing(mapped, configurationPropertyName.toString(ToStringFormat.LEGACY_SYSTEM_ENVIRONMENT, false));
+		return mapped;
+	}
+
+	private void addIfMissing(List<String> list, String value) {
+		if (!list.contains(value)) {
+			list.add(value);
+		}
 	}
 
 	@Override

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/source/SystemEnvironmentPropertyMapperTests.java

@@ -48,13 +48,14 @@ class SystemEnvironmentPropertyMapperTests extends AbstractPropertyMapperTests {
 
 	@Test
 	void mapFromConfigurationShouldReturnBestGuess() {
-		assertThat(mapConfigurationPropertyName("server")).containsExactly("SERVER");
-		assertThat(mapConfigurationPropertyName("server.port")).containsExactly("SERVER_PORT");
-		assertThat(mapConfigurationPropertyName("host[0]")).containsExactly("HOST_0");
-		assertThat(mapConfigurationPropertyName("host[0][1]")).containsExactly("HOST_0_1");
-		assertThat(mapConfigurationPropertyName("host[0].name")).containsExactly("HOST_0_NAME");
-		assertThat(mapConfigurationPropertyName("host.f00.name")).containsExactly("HOST_F00_NAME");
-		assertThat(mapConfigurationPropertyName("foo.the-bar")).containsExactly("FOO_THEBAR", "FOO_THE_BAR");
+		assertThat(mapConfigurationPropertyName("server")).containsExactly("SERVER", "server");
+		assertThat(mapConfigurationPropertyName("server.port")).containsExactly("SERVER_PORT", "server_port");
+		assertThat(mapConfigurationPropertyName("host[0]")).containsExactly("HOST_0", "host_0");
+		assertThat(mapConfigurationPropertyName("host[0][1]")).containsExactly("HOST_0_1", "host_0_1");
+		assertThat(mapConfigurationPropertyName("host[0].name")).containsExactly("HOST_0_NAME", "host_0_name");
+		assertThat(mapConfigurationPropertyName("host.f00.name")).containsExactly("HOST_F00_NAME", "host_f00_name");
+		assertThat(mapConfigurationPropertyName("foo.the-bar")).containsExactly("FOO_THEBAR", "FOO_THE_BAR",
+				"foo_thebar", "foo_the_bar");
 	}
 
 	@Test