Polish "Improve performance of CompositePropertySource#getPropertyNames"

See gh-27236

spring-core/src/test/java/org/springframework/core/env/CompositePropertySourceTests.java

@@ -17,6 +17,7 @@
 package org.springframework.core.env;
 
 import java.util.Collections;
+import java.util.Map;
 
 import org.junit.jupiter.api.Test;
 
@@ -46,4 +47,13 @@ class CompositePropertySourceTests {
 		assertThat(((i1 < i2) && (i2 < i3))).as("Bad order: " + s).isTrue();
 	}
 
+	@Test
+	void getPropertyNamesRemovesDuplicates() {
+		CompositePropertySource composite = new CompositePropertySource("c");
+		composite.addPropertySource(new MapPropertySource("p1", Map.of("p1.property", "value")));
+		composite.addPropertySource(new MapPropertySource("p2",
+				Map.of("p2.property1", "value", "p1.property", "value", "p2.property2", "value")));
+		assertThat(composite.getPropertyNames()).containsOnly("p1.property", "p2.property1", "p2.property2");
+	}
+
 }